Implement Marked Nether Portal System

- Introduced MarkedNetherPortal and MarkedNetherPortals classes to manage the marking and activation of Nether portals.
- Added PortalMarkStore for persistent storage of marked portals in YAML format.
- Created NetherPortalIgniter for a special item that allows marking portals in creative mode.
- Enhanced NetherAccess and NetherAccessListener to integrate the new portal marking system.
- Implemented commands and event handling for marking portals and managing their states.
- Added validation and chunk loading mechanisms to ensure proper portal functionality.

common/src/main/java/eu/mhsl/craftattack/spawn/common/appliances/tooling/dimensionScheduler/README.md

@@ -9,6 +9,9 @@
 | `DimensionScheduleText` | End-Grün (`#80A71A`), Nether-Rot (`#B83D32`) und deutsche Formatierung von Restzeiten und Terminen. Absolute Zeiten werden in `Europe/Berlin` mit Zeitzone ausgegeben. |
 | `EndPrevent` | Besitzt die End-Effektausgabe. Erstellt in `onEnable()` eine `EndOpeningSchedule`, wenn die Scheduler-Appliance vorhanden ist, und stoppt sie in `onDisable()`. Verwaltet End-Zustand und Weltgrenze. |
 | `NetherAccess` | Neue Appliance in `common`. Besitzt die Nether-Effektausgabe, erstellt und stoppt `NetherOpeningSchedule` und verwaltet den Nether-Zustand. Die bisherigen Varo-Klassen bleiben unverändert und werden hierfür nicht eingebunden. |
+| `NetherPortalIgniter` / `NetherIgniterCommand` | Erstellen das eindeutig markierte Portalfeuerzeug und geben es über `/netherIgniter` an OPs im Kreativmodus aus. |
+| `MarkedNetherPortals` | Wird von `NetherAccess` erstellt. Verwaltet Markierungen, Chunk-Index und die vorübergehende Aktivierungswarteschlange auf dem Hauptthread. |
+| `MarkedNetherPortal` / `PortalMarkStore` | Unveränderliche Portalgeometrie ohne gespeicherte Welt-/Block-/Spielerobjekte und serialisierte Hintergrundspeicherung in einer eigenen YAML. |
 | `EndOpeningSchedule` / `NetherOpeningSchedule` | Je ein Bukkit-Task. Starten im nächsten Tick nach der Initialisierung aller Appliances und prüfen dann alle 20 Ticks Termine und Ankündigungen. Die Zustandslogik bleibt bei der jeweiligen Dimension; End behält seine Outer-End-Logik. |
 | `DimensionOpeningEffects` | Gemeinsame Nachrichten- und Soundausgabe mit übergebenen Konfigurationsgruppen und Farbe. Wählt Ablehnungstexte zufällig aus und gibt Effekte über `play(effect)` aus. Den Nachrichten-Cooldown verwaltet der jeweilige Portal-Listener. |
 | `OpeningAnnouncements` | Gemeinsame Verfolgung fälliger und übersprungener Vorankündigungen. Verwendet die dynamischen Effekte, ohne Zustände zu ändern oder eigene Tasks anzulegen. |
@@ -179,15 +182,65 @@ Entfernung zum Portal ermittelt. Nachrichten und Sounds teilen einen Cooldown vo
 Sekunden pro Spieler. Die Sperre gilt auch während des Cooldowns; Spieler-Einträge werden beim
 Verlassen entfernt. Alle Nether-Ausgaben verwenden Nether-Rot (`#B83D32`).
 
+## Portalfeuerzeug und vorgemerkte Netherportale
+
+OPs im Kreativmodus erhalten mit `/netherIgniter` ein glänzendes Feuerzeug namens
+„Portalfeuerzeug“ mit erklärender Lore. Bei vollem Inventar schlägt der Befehl fehl;
+es wird kein Item fallen gelassen. Ein PersistentDataContainer-Marker identifiziert das
+Item unabhängig vom Anzeigenamen. Das Umbenennen eines normalen Feuerzeugs genügt nicht.
+Auch ein weitergegebenes Portalfeuerzeug lässt sich ausschließlich im Kreativmodus benutzen.
+
+Bei offenem Nether und für Rückportale innerhalb des Nethers bleibt die gewöhnliche
+Vanilla-Benutzung erhalten. Ist der Nether geschlossen, kann das Spezialitem gültige
+Zugangsportale außerhalb des Nethers vormerken: Der Listener erfasst die tatsächlich
+verwendete Hand, Position und den Server-Tick. Erst das zugehörige native
+`PortalCreateEvent(FIRE)` liefert den geprüften Rahmen. Die Portalbildung wird abgebrochen
+und das zurückgebliebene Zündfeuer im nächsten Tick gezielt entfernt. Statt einer
+Ablehnungsnachricht erhält nur der Spieler eine Bestätigung in Nether-Rot, sobald die
+Markierung erfolgreich gespeichert wurde. Mehrfaches Vormerken erzeugt keinen zweiten Eintrag.
+
+`plugins/spawn/marked-nether-portals.yml` enthält Welt- und Ersteller-UUID, Innenraum,
+Achse, Rahmen und Zündposition. Die Datei entsteht beim ersten Speichern einer Markierung.
+Ein eigener serieller Hintergrund-Worker schreibt Änderungen zuerst in eine temporäre
+Datei und ersetzt anschließend die Speicherdatei. Fehlgeschlagene Schreibvorgänge werden
+nicht als erfolgreiche Markierung bestätigt. Eine beschädigte Speicherdatei wird weder
+überschrieben noch teilweise geladen; neue Markierungen bleiben bis zur Behebung und
+erneuten Aktivierung der Appliance gesperrt. Die Termin- und Effekt-YAML bleibt unverändert.
+
+Der gemeinsame Zustandswechsel von `NetherAccess` startet vorgemerkte Portale bei Öffnung
+per Command, Zeitplan und stiller Wiederherstellung. Beim Serverstart werden gespeicherte
+Markierungen erst nach der anfänglichen Terminprüfung berücksichtigt. Dadurch funktioniert
+auch ein bereits gespeichert offener Nether, ohne vergangene Öffnungsnachrichten abzuspielen.
+
+Es werden keine Chunks geladen oder erzeugt. Ein Chunk-Index ordnet reguläre
+`ChunkLoadEvent`s den betroffenen Portalen zu. Erst wenn alle Chunks des Rahmens und des
+benötigten Vanilla-Suchbereichs geladen sind, wird das Portal zur Aktivierung eingereiht.
+Ungeladene Portale bleiben gespeichert und öffnen entsprechend später, wenn ihre Umgebung
+regulär geladen wird. Die Warteschlange ist dedupliziert und bearbeitet höchstens ein Portal
+pro Tick. Ihr Task läuft ausschließlich bei gefüllter Warteschlange; Schließen des Nethers
+oder Deaktivieren der Appliance stoppt die Verarbeitung.
+
+Vor der Zündung werden Zugangszustand, geladene Chunks, Obsidianrahmen und freier Innenraum
+erneut geprüft. Beschädigte oder verbaute Portale werden mit protokollierter Ursache aus der
+Registrierung entfernt; fremde Blöcke werden nicht ersetzt. Die Zündung setzt Feuer mit
+Physik und überlässt die Portalbildung Vanilla. Abgebrochene `BlockIgniteEvent`s und native
+`PortalCreateEvent`s anderer Plugins werden respektiert: Die Markierung bleibt gespeichert,
+der Grund wird protokolliert. Es gibt keinen laufenden Wiederholungsversuch; erst erneutes
+reguläres Laden der Umgebung oder eine weitere Nether-Öffnung kann das Portal erneut einreihen.
+Erfolg wird an den tatsächlichen Portalblöcken geprüft. Erfolgreich aktivierte sowie bereits
+vollständig aktive Portale werden aus der Speicherdatei entfernt.
+
 ## Erweiterungen und Leistung
 
 Weitere Effektarten können später zentral in `DimensionOpeningEffects.play(effect)`
 ergänzt werden und werden bei manueller sowie zeitgesteuerter Öffnung gleichermaßen ausgeführt.
 Neue Ankündigungen benötigen nur einen weiteren YAML-Eintrag mit Minutenwert und Text.
 
-Es bleibt bei einem Task pro Dimension mit wenigen Zeitvergleichen pro Sekunde und je einer
+Für Termine bleibt es bei einem Task pro Dimension mit wenigen Zeitvergleichen pro Sekunde und je einer
 kleinen Menge behandelter Effektnamen. Es entsteht kein allgemeines Scheduler-Framework.
 Die Prüfung arbeitet im Arbeitsspeicher. Chat,
 Zustandswechsel und spätere Bukkit-Effekte laufen auf dem Server-Hauptthread. Gespeichert
 wird nur bei tatsächlichen Zustandsänderungen in `config.yml`; die Termin-YAML wird nach
-ihrer erstmaligen Erstellung ausschließlich gelesen. Es gibt keine persistenten Ausführungsstatus.
+ihrer erstmaligen Erstellung ausschließlich gelesen. Es gibt keine persistenten Ausführungsstatus
+für Ankündigungen. Portalmarkierungen verwenden davon getrennt die oben beschriebene Datei und
+begrenzte Warteschlange; ihre Dateioperationen laufen außerhalb des Server-Hauptthreads.

common/src/main/java/eu/mhsl/craftattack/spawn/common/appliances/tooling/netherAccess/MarkedNetherPortal.java

@@ -0,0 +1,232 @@
+package eu.mhsl.craftattack.spawn.common.appliances.tooling.netherAccess;
+
+import org.bukkit.Axis;
+import org.bukkit.Material;
+import org.bukkit.World;
+import org.bukkit.block.Block;
+import org.bukkit.block.BlockState;
+import org.bukkit.block.data.Orientable;
+import org.bukkit.entity.Player;
+import org.bukkit.event.world.PortalCreateEvent;
+
+import java.util.ArrayList;
+import java.util.HashSet;
+import java.util.LinkedHashSet;
+import java.util.List;
+import java.util.Objects;
+import java.util.Set;
+import java.util.UUID;
+
+/** An immutable portal shape; no blocks, worlds or players survive the marking event. */
+record MarkedNetherPortal(
+    UUID worldId,
+    UUID creatorId,
+    Position origin,
+    int width,
+    int height,
+    Axis axis,
+    Position ignition,
+    List<Position> frame
+) {
+    private static final int NATIVE_SEARCH_RADIUS = 22;
+
+    record Position(int x, int y, int z) {
+        Block block(World world) {
+            return world.getBlockAt(this.x, this.y, this.z);
+        }
+    }
+
+    record ChunkKey(UUID worldId, int x, int z) {
+    }
+
+    enum Validation {
+        WAITING,
+        INVALID,
+        READY,
+        ALREADY_ACTIVE
+    }
+
+    MarkedNetherPortal {
+        Objects.requireNonNull(worldId, "worldId");
+        Objects.requireNonNull(creatorId, "creatorId");
+        Objects.requireNonNull(origin, "origin");
+        Objects.requireNonNull(ignition, "ignition");
+        Objects.requireNonNull(frame, "frame");
+        if(axis != Axis.X && axis != Axis.Z) throw new IllegalArgumentException("Invalid portal axis");
+        if(width < 2 || width > 21 || height < 3 || height > 21) {
+            throw new IllegalArgumentException("Invalid portal dimensions");
+        }
+
+        List<Position> expectedFrame;
+        try {
+            expectedFrame = framePositions(origin, width, height, axis);
+            Math.subtractExact(ignition.x, NATIVE_SEARCH_RADIUS);
+            Math.addExact(ignition.x, NATIVE_SEARCH_RADIUS);
+            Math.subtractExact(ignition.z, NATIVE_SEARCH_RADIUS);
+            Math.addExact(ignition.z, NATIVE_SEARCH_RADIUS);
+        } catch(ArithmeticException exception) {
+            throw new IllegalArgumentException("Portal coordinates overflow", exception);
+        }
+        if(!contains(origin, width, height, axis, ignition)) {
+            throw new IllegalArgumentException("Ignition position is outside the portal");
+        }
+        if(frame.size() != expectedFrame.size() || !new HashSet<>(frame).equals(new HashSet<>(expectedFrame))) {
+            throw new IllegalArgumentException("Frame positions do not match the portal perimeter");
+        }
+        frame = List.copyOf(expectedFrame);
+    }
+
+    String id() {
+        return this.worldId + ":" + this.origin.x + ":" + this.origin.y + ":" + this.origin.z
+            + ":" + this.axis + ":" + this.width + ":" + this.height;
+    }
+
+    /** Uses proposed block snapshots, after Vanilla has already recognised a valid frame. */
+    static MarkedNetherPortal from(PortalCreateEvent event, Player creator, Position ignition) {
+        if(event.getReason() != PortalCreateEvent.CreateReason.FIRE) {
+            throw new IllegalArgumentException("Only fire-created portals can be marked");
+        }
+
+        Set<Position> interior = new HashSet<>();
+        Set<Position> obsidian = new HashSet<>();
+        Axis axis = null;
+        for(BlockState block : event.getBlocks()) {
+            if(!block.getWorld().getUID().equals(event.getWorld().getUID())) {
+                throw new IllegalArgumentException("Portal snapshots belong to a different world");
+            }
+            Position position = new Position(block.getX(), block.getY(), block.getZ());
+            if(block.getType() == Material.OBSIDIAN) obsidian.add(position);
+            if(block.getType() != Material.NETHER_PORTAL) continue;
+            if(!(block.getBlockData() instanceof Orientable orientable)) {
+                throw new IllegalArgumentException("Portal snapshot has no axis");
+            }
+            Axis blockAxis = orientable.getAxis();
+            if(blockAxis != Axis.X && blockAxis != Axis.Z) {
+                throw new IllegalArgumentException("Invalid portal snapshot axis");
+            }
+            if(axis != null && axis != blockAxis) throw new IllegalArgumentException("Mixed portal axes");
+            axis = blockAxis;
+            if(!interior.add(position)) throw new IllegalArgumentException("Duplicate portal snapshot");
+        }
+        if(interior.isEmpty()) throw new IllegalArgumentException("No proposed portal blocks");
+
+        int minX = interior.stream().mapToInt(Position::x).min().orElseThrow();
+        int maxX = interior.stream().mapToInt(Position::x).max().orElseThrow();
+        int minY = interior.stream().mapToInt(Position::y).min().orElseThrow();
+        int maxY = interior.stream().mapToInt(Position::y).max().orElseThrow();
+        int minZ = interior.stream().mapToInt(Position::z).min().orElseThrow();
+        int maxZ = interior.stream().mapToInt(Position::z).max().orElseThrow();
+        if((axis == Axis.X && minZ != maxZ) || (axis == Axis.Z && minX != maxX)) {
+            throw new IllegalArgumentException("Portal snapshots do not form a plane");
+        }
+        long proposedWidth = axis == Axis.X ? (long) maxX - minX + 1 : (long) maxZ - minZ + 1;
+        long proposedHeight = (long) maxY - minY + 1;
+        if(proposedWidth < 2 || proposedWidth > 21 || proposedHeight < 3 || proposedHeight > 21
+            || interior.size() != proposedWidth * proposedHeight) {
+            throw new IllegalArgumentException("Portal snapshots do not form a valid rectangle");
+        }
+        int width = (int) proposedWidth;
+        int height = (int) proposedHeight;
+
+        Position origin = new Position(minX, minY, minZ);
+        for(int horizontal = 0; horizontal < width; horizontal++) {
+            for(int vertical = 0; vertical < height; vertical++) {
+                if(!interior.contains(offset(origin, horizontal, vertical, axis))) {
+                    throw new IllegalArgumentException("Portal snapshots contain a hole");
+                }
+            }
+        }
+        List<Position> frame = framePositions(origin, width, height, axis);
+        if(!obsidian.containsAll(frame)) throw new IllegalArgumentException("Missing obsidian frame snapshots");
+        return new MarkedNetherPortal(
+            event.getWorld().getUID(), creator.getUniqueId(), origin, width, height, axis, ignition, frame
+        );
+    }
+
+    Set<ChunkKey> requiredChunks() {
+        Set<ChunkKey> chunks = new LinkedHashSet<>();
+        // Native fire placement searches neighbouring possible frames, not only this exact rectangle.
+        this.addChunkRectangle(
+            chunks,
+            this.ignition.x - NATIVE_SEARCH_RADIUS, this.ignition.x + NATIVE_SEARCH_RADIUS,
+            this.ignition.z - NATIVE_SEARCH_RADIUS, this.ignition.z + NATIVE_SEARCH_RADIUS
+        );
+        for(Position position : this.frame) {
+            chunks.add(new ChunkKey(this.worldId, Math.floorDiv(position.x, 16), Math.floorDiv(position.z, 16)));
+        }
+        return Set.copyOf(chunks);
+    }
+
+    boolean isLoaded(World world) {
+        if(world == null || !world.getUID().equals(this.worldId)) return false;
+        for(ChunkKey chunk : this.requiredChunks()) {
+            if(!world.isChunkLoaded(chunk.x, chunk.z)) return false;
+        }
+        return true;
+    }
+
+    /** Must run on the main thread; no block is read until every required chunk is present. */
+    Validation validate(World world) {
+        if(!this.isLoaded(world)) return Validation.WAITING;
+        if(this.origin.y - 1 < world.getMinHeight() || this.origin.y + this.height >= world.getMaxHeight()) {
+            return Validation.INVALID;
+        }
+        for(Position position : this.frame) {
+            if(position.block(world).getType() != Material.OBSIDIAN) return Validation.INVALID;
+        }
+
+        int active = 0;
+        for(int horizontal = 0; horizontal < this.width; horizontal++) {
+            for(int vertical = 0; vertical < this.height; vertical++) {
+                Block block = offset(this.origin, horizontal, vertical, this.axis).block(world);
+                Material type = block.getType();
+                if(type == Material.NETHER_PORTAL) {
+                    if(!(block.getBlockData() instanceof Orientable data) || data.getAxis() != this.axis) {
+                        return Validation.INVALID;
+                    }
+                    active++;
+                } else if(!type.isAir() && type != Material.FIRE) {
+                    return Validation.INVALID;
+                }
+            }
+        }
+        if(active == this.width * this.height) return Validation.ALREADY_ACTIVE;
+        return active == 0 ? Validation.READY : Validation.INVALID;
+    }
+
+    private void addChunkRectangle(Set<ChunkKey> chunks, int minX, int maxX, int minZ, int maxZ) {
+        for(int x = Math.floorDiv(minX, 16); x <= Math.floorDiv(maxX, 16); x++) {
+            for(int z = Math.floorDiv(minZ, 16); z <= Math.floorDiv(maxZ, 16); z++) {
+                chunks.add(new ChunkKey(this.worldId, x, z));
+            }
+        }
+    }
+
+    private static boolean contains(Position origin, int width, int height, Axis axis, Position position) {
+        long horizontal = axis == Axis.X ? (long) position.x - origin.x : (long) position.z - origin.z;
+        long vertical = (long) position.y - origin.y;
+        boolean samePlane = axis == Axis.X ? position.z == origin.z : position.x == origin.x;
+        return samePlane && horizontal >= 0 && horizontal < width && vertical >= 0 && vertical < height;
+    }
+
+    private static List<Position> framePositions(Position origin, int width, int height, Axis axis) {
+        List<Position> positions = new ArrayList<>(2 * (width + height));
+        for(int horizontal = 0; horizontal < width; horizontal++) {
+            positions.add(offset(origin, horizontal, -1, axis));
+            positions.add(offset(origin, horizontal, height, axis));
+        }
+        for(int vertical = 0; vertical < height; vertical++) {
+            positions.add(offset(origin, -1, vertical, axis));
+            positions.add(offset(origin, width, vertical, axis));
+        }
+        return positions;
+    }
+
+    private static Position offset(Position origin, int horizontal, int vertical, Axis axis) {
+        return new Position(
+            Math.addExact(origin.x, axis == Axis.X ? horizontal : 0),
+            Math.addExact(origin.y, vertical),
+            Math.addExact(origin.z, axis == Axis.Z ? horizontal : 0)
+        );
+    }
+}

common/src/main/java/eu/mhsl/craftattack/spawn/common/appliances/tooling/netherAccess/MarkedNetherPortals.java

@@ -0,0 +1,284 @@
+package eu.mhsl.craftattack.spawn.common.appliances.tooling.netherAccess;
+
+import eu.mhsl.craftattack.spawn.common.appliances.tooling.dimensionScheduler.DimensionScheduleText;
+import eu.mhsl.craftattack.spawn.core.Main;
+import org.bukkit.Bukkit;
+import org.bukkit.Chunk;
+import org.bukkit.Material;
+import org.bukkit.World;
+import org.bukkit.block.Block;
+import org.bukkit.entity.Player;
+import org.bukkit.event.block.BlockIgniteEvent;
+import org.bukkit.event.world.PortalCreateEvent;
+import org.bukkit.plugin.IllegalPluginAccessException;
+import org.bukkit.scheduler.BukkitTask;
+import org.jetbrains.annotations.Nullable;
+
+import java.util.HashMap;
+import java.util.HashSet;
+import java.util.LinkedHashMap;
+import java.util.LinkedHashSet;
+import java.util.Map;
+import java.util.Set;
+import java.util.UUID;
+import java.util.logging.Level;
+
+/** Main-thread registry and bounded activation queue; only the store touches its YAML. */
+final class MarkedNetherPortals {
+    private final NetherAccess owner;
+    private final PortalMarkStore store;
+    private final Map<String, MarkedNetherPortal> portals = new LinkedHashMap<>();
+    private final Map<MarkedNetherPortal.ChunkKey, Set<String>> chunkIndex = new HashMap<>();
+    private final Set<String> removing = new HashSet<>();
+    private final LinkedHashSet<String> queue = new LinkedHashSet<>();
+    private @Nullable BukkitTask processingTask;
+    private @Nullable BukkitTask startupTask;
+    private boolean loaded;
+    private boolean loadFailed;
+    private boolean ready;
+    private volatile boolean closed;
+
+    MarkedNetherPortals(NetherAccess owner) {
+        this.owner = owner;
+        this.store = new PortalMarkStore(Main.instance().getDataFolder().toPath().resolve("marked-nether-portals.yml"));
+    }
+
+    void start() {
+        this.store.load().whenComplete((saved, error) -> {
+            if(error != null) {
+                this.logStorageFailure("Portalmarkierungen konnten nicht geladen werden; neue Markierungen sind gesperrt.", error);
+                this.onMainThread(() -> this.loadFailed = true);
+                return;
+            }
+            this.onMainThread(() -> {
+                saved.forEach(this::register);
+                this.loaded = true;
+                this.enqueueLoadedPortals();
+            });
+        });
+        // The opening schedule reconciles its initial state on tick one. In particular,
+        // a persisted OPEN must not ignite portals before a future date restores CLOSED.
+        this.startupTask = Bukkit.getScheduler().runTaskLater(Main.instance(), () -> {
+            this.startupTask = null;
+            this.ready = true;
+            this.enqueueLoadedPortals();
+        }, 2L);
+    }
+
+    void mark(PortalCreateEvent event, Player player, MarkedNetherPortal.Position ignition) {
+        UUID playerId = player.getUniqueId();
+        UUID worldId = event.getWorld().getUID();
+        // Cancellation leaves the source fire behind. Remove only that exact fire,
+        // after the original native ignition has returned, without touching other blocks.
+        Bukkit.getScheduler().runTask(Main.instance(), () -> {
+            if(this.closed) return;
+            World world = Bukkit.getWorld(worldId);
+            if(world == null || !world.isChunkLoaded(ignition.x() >> 4, ignition.z() >> 4)) return;
+            Block block = ignition.block(world);
+            if(block.getType() == Material.FIRE) block.setType(Material.AIR, false);
+        });
+        if(!this.loaded) {
+            this.message(playerId, this.loadFailed
+                ? "Die Portal-Speicherdatei konnte nicht geladen werden. Bitte informiere einen Administrator. Das Portal wurde nicht vorgemerkt."
+                : "Die Portalmarkierungen werden noch geladen. Bitte versuche es gleich erneut.");
+            return;
+        }
+
+        final MarkedNetherPortal portal;
+        try {
+            portal = MarkedNetherPortal.from(event, player, ignition);
+        } catch(IllegalArgumentException error) {
+            this.message(playerId, "Dieses Portal konnte nicht vorgemerkt werden.");
+            Main.logger().log(Level.WARNING, "Ungültige Geometrie beim Vormerken eines Netherportals.", error);
+            return;
+        }
+        this.store.putIfAbsent(portal).whenComplete((saved, error) -> {
+            if(error != null) {
+                this.logStorageFailure("Netherportal " + portal.id() + " konnte nicht gespeichert werden.", error);
+                this.onMainThread(() -> this.message(playerId, "Das Portal konnte nicht gespeichert werden und wurde nicht vorgemerkt. Bitte informiere einen Administrator."));
+                return;
+            }
+            this.onMainThread(() -> {
+                boolean existing = this.portals.containsKey(saved.id());
+                this.register(saved);
+                this.message(playerId, existing
+                    ? "Dieses Portal ist bereits vorgemerkt und wird nach der Nether-Freigabe entzündet."
+                    : "Das Portal wurde vorgemerkt und wird nach der Nether-Freigabe automatisch entzündet.");
+                this.enqueue(saved);
+            });
+        });
+    }
+
+    /** Called for both live transitions and silent schedule restoration. */
+    void onStateChanged() {
+        if(this.owner.isClosed()) {
+            this.queue.clear();
+            this.stopProcessing();
+        } else {
+            this.enqueueLoadedPortals();
+        }
+    }
+
+    void onChunkLoad(Chunk chunk) {
+        if(!this.canProcess()) return;
+        Set<String> affected = this.chunkIndex.get(new MarkedNetherPortal.ChunkKey(chunk.getWorld().getUID(), chunk.getX(), chunk.getZ()));
+        if(affected == null) return;
+        affected.forEach(id -> this.enqueue(this.portals.get(id)));
+    }
+
+    void onWorldLoad(World world) {
+        if(!this.canProcess()) return;
+        this.portals.values().stream().filter(portal -> portal.worldId().equals(world.getUID())).forEach(this::enqueue);
+    }
+
+    private void register(MarkedNetherPortal portal) {
+        if(this.portals.putIfAbsent(portal.id(), portal) != null) return;
+        for(MarkedNetherPortal.ChunkKey chunk : portal.requiredChunks()) {
+            this.chunkIndex.computeIfAbsent(chunk, ignored -> new HashSet<>()).add(portal.id());
+        }
+    }
+
+    private boolean canProcess() {
+        return !this.closed && this.loaded && this.ready && !this.owner.isClosed();
+    }
+
+    private void enqueueLoadedPortals() {
+        if(this.canProcess()) this.portals.values().forEach(this::enqueue);
+    }
+
+    private void enqueue(@Nullable MarkedNetherPortal portal) {
+        if(portal == null || !this.canProcess() || this.removing.contains(portal.id())) return;
+        World world = Bukkit.getWorld(portal.worldId());
+        if(world == null || !portal.isLoaded(world)) return;
+        this.queue.add(portal.id());
+        if(this.processingTask == null) {
+            this.processingTask = Bukkit.getScheduler().runTaskTimer(Main.instance(), this::processNext, 1L, 1L);
+        }
+    }
+
+    private void processNext() {
+        if(!this.canProcess()) {
+            this.queue.clear();
+            this.stopProcessing();
+            return;
+        }
+        if(this.queue.isEmpty()) {
+            this.stopProcessing();
+            return;
+        }
+        String id = this.queue.removeFirst();
+        MarkedNetherPortal portal = this.portals.get(id);
+        World world = portal == null ? null : Bukkit.getWorld(portal.worldId());
+        if(portal != null && world != null && !this.removing.contains(id)) {
+            switch(portal.validate(world)) {
+                case WAITING -> { /* A required chunk unloaded while this entry was queued. */ }
+                case INVALID -> this.discard(portal, "Rahmen beschädigt oder Innenraum verbaut");
+                case ALREADY_ACTIVE -> this.remove(portal);
+                case READY -> this.ignite(portal, world);
+            }
+        }
+        if(this.queue.isEmpty()) this.stopProcessing();
+    }
+
+    private void ignite(MarkedNetherPortal portal, World world) {
+        Block ignition = portal.ignition().block(world);
+        Player creator = Bukkit.getPlayer(portal.creatorId());
+        BlockIgniteEvent igniteEvent = new BlockIgniteEvent(ignition, BlockIgniteEvent.IgniteCause.FLINT_AND_STEEL, creator);
+        Bukkit.getPluginManager().callEvent(igniteEvent);
+        if(igniteEvent.isCancelled()) {
+            this.logRetained(portal, "BlockIgniteEvent abgebrochen");
+            return;
+        }
+        // Other plugins may have changed access, blocks or chunk availability in the event.
+        if(!this.canProcess()) return;
+        switch(portal.validate(world)) {
+            case WAITING -> { return; }
+            case INVALID -> { this.discard(portal, "Portal während des Zündereignisses verändert"); return; }
+            case ALREADY_ACTIVE -> { this.remove(portal); return; }
+            case READY -> { }
+        }
+
+        boolean hadFire = ignition.getType() == Material.FIRE;
+        if(hadFire) ignition.setType(Material.AIR, false);
+        // Native physics performs shape validation and fires PortalCreateEvent. Writing
+        // portal blocks ourselves would bypass cancellation by protection plugins.
+        ignition.setType(Material.FIRE, true);
+        MarkedNetherPortal.Validation result = portal.validate(world);
+        if(result == MarkedNetherPortal.Validation.ALREADY_ACTIVE) {
+            this.remove(portal);
+        } else {
+            if(!hadFire && world.isChunkLoaded(ignition.getX() >> 4, ignition.getZ() >> 4)
+                && ignition.getType() == Material.FIRE) ignition.setType(Material.AIR, false);
+            if(result == MarkedNetherPortal.Validation.INVALID) {
+                this.discard(portal, "Portal beim Zünden beschädigt oder verbaut");
+            } else {
+                this.logRetained(portal, "Vanilla-Portalbildung nicht erfolgt oder von einem Plugin abgebrochen");
+            }
+        }
+    }
+
+    private void discard(MarkedNetherPortal portal, String reason) {
+        Main.logger().warning("Netherportal-Markierung " + portal.id() + " wird entfernt: " + reason + ".");
+        this.remove(portal);
+    }
+
+    private void logRetained(MarkedNetherPortal portal, String reason) {
+        Main.logger().warning("Netherportal " + portal.id() + " bleibt vorgemerkt: " + reason
+            + ". Kein automatischer Wiederholungsversuch ohne erneutes Laden oder Öffnen.");
+    }
+
+    private void remove(MarkedNetherPortal portal) {
+        String id = portal.id();
+        if(!this.removing.add(id)) return;
+        this.store.remove(id).whenComplete((ignored, error) -> {
+            if(error != null) this.logStorageFailure("Netherportal-Markierung " + id + " konnte nicht entfernt werden.", error);
+            this.onMainThread(() -> {
+                this.removing.remove(id);
+                if(error != null) return;
+                this.portals.remove(id);
+                this.queue.remove(id);
+                for(MarkedNetherPortal.ChunkKey chunk : portal.requiredChunks()) {
+                    Set<String> entries = this.chunkIndex.get(chunk);
+                    if(entries == null) continue;
+                    entries.remove(id);
+                    if(entries.isEmpty()) this.chunkIndex.remove(chunk);
+                }
+            });
+        });
+    }
+
+    private void message(UUID playerId, String text) {
+        Player player = Bukkit.getPlayer(playerId);
+        if(player != null) player.sendMessage(DimensionScheduleText.message(text, DimensionScheduleText.NETHER_RED));
+    }
+
+    private void onMainThread(Runnable action) {
+        if(this.closed) return;
+        try {
+            Bukkit.getScheduler().runTask(Main.instance(), () -> {
+                if(!this.closed) action.run();
+            });
+        } catch(IllegalPluginAccessException ignored) {
+            // Persisted changes survive shutdown even when no main-thread callback can run.
+        }
+    }
+
+    private void logStorageFailure(String message, Throwable error) {
+        Main.logger().log(Level.SEVERE, message, error);
+    }
+
+    private void stopProcessing() {
+        if(this.processingTask == null) return;
+        this.processingTask.cancel();
+        this.processingTask = null;
+    }
+
+    void close() {
+        this.closed = true;
+        if(this.startupTask != null) this.startupTask.cancel();
+        this.startupTask = null;
+        this.stopProcessing();
+        this.queue.clear();
+        this.store.close();
+    }
+}

common/src/main/java/eu/mhsl/craftattack/spawn/common/appliances/tooling/netherAccess/NetherAccess.java

@@ -12,6 +12,7 @@ import org.jetbrains.annotations.NotNull;
 import org.jetbrains.annotations.Nullable;
 
 import java.util.List;
+import java.util.Objects;
 
 /** Owns Nether access; commands and the schedule use the same persisted state transition. */
 public class NetherAccess extends Appliance {
@@ -20,6 +21,7 @@ public class NetherAccess extends Appliance {
     );
     private State state;
     private @Nullable NetherOpeningSchedule openingSchedule;
+    private @Nullable MarkedNetherPortals markedPortals;
 
     public enum State {
         CLOSED,
@@ -34,6 +36,9 @@ public class NetherAccess extends Appliance {
     @Override
     public void onEnable() {
         if(this.openingSchedule != null) this.openingSchedule.stop();
+        if(this.markedPortals != null) this.markedPortals.close();
+        this.markedPortals = new MarkedNetherPortals(this);
+        this.markedPortals.start();
         DimensionScheduler scheduler = Main.instance().getAppliances().stream()
             .filter(DimensionScheduler.class::isInstance)
             .map(DimensionScheduler.class::cast)
@@ -48,6 +53,7 @@ public class NetherAccess extends Appliance {
     @Override
     public void onDisable() {
         if(this.openingSchedule != null) this.openingSchedule.stop();
+        if(this.markedPortals != null) this.markedPortals.close();
     }
 
     public void setState(State state) {
@@ -65,6 +71,7 @@ public class NetherAccess extends Appliance {
         this.state = state;
         this.localConfig().set("state", state.name());
         Configuration.saveChanges();
+        if(this.markedPortals != null) this.markedPortals.onStateChanged();
         if(announceOpening && wasClosed && !this.isClosed()) this.openingEffects.playOpening();
     }
 
@@ -80,6 +87,10 @@ public class NetherAccess extends Appliance {
         return this.openingEffects;
     }
 
+    MarkedNetherPortals markedPortals() {
+        return Objects.requireNonNull(this.markedPortals, "Nether portal registry is not enabled");
+    }
+
     @Override
     protected @NotNull List<Listener> listeners() {
         return List.of(new NetherAccessListener());
@@ -87,6 +98,6 @@ public class NetherAccess extends Appliance {
 
     @Override
     protected @NotNull List<ApplianceCommand<?>> commands() {
-        return List.of(new NetherCommand(), new NetherPreventCommand());
+        return List.of(new NetherCommand(), new NetherPreventCommand(), new NetherIgniterCommand());
     }
 }

common/src/main/java/eu/mhsl/craftattack/spawn/common/appliances/tooling/netherAccess/NetherAccessListener.java

@@ -1,16 +1,28 @@
 package eu.mhsl.craftattack.spawn.common.appliances.tooling.netherAccess;
 
+import eu.mhsl.craftattack.spawn.common.appliances.tooling.dimensionScheduler.DimensionScheduleText;
 import eu.mhsl.craftattack.spawn.core.appliance.ApplianceListener;
+import org.bukkit.Bukkit;
+import org.bukkit.GameMode;
 import org.bukkit.Location;
+import org.bukkit.Material;
 import org.bukkit.World;
+import org.bukkit.block.Block;
 import org.bukkit.entity.Entity;
 import org.bukkit.entity.Player;
 import org.bukkit.event.Cancellable;
+import org.bukkit.event.Event;
 import org.bukkit.event.EventHandler;
+import org.bukkit.event.EventPriority;
+import org.bukkit.event.block.Action;
 import org.bukkit.event.entity.EntityPortalEvent;
+import org.bukkit.event.player.PlayerInteractEvent;
 import org.bukkit.event.player.PlayerPortalEvent;
 import org.bukkit.event.player.PlayerQuitEvent;
+import org.bukkit.event.world.ChunkLoadEvent;
 import org.bukkit.event.world.PortalCreateEvent;
+import org.bukkit.event.world.WorldLoadEvent;
+import org.bukkit.inventory.EquipmentSlot;
 import org.jetbrains.annotations.Nullable;
 
 import java.util.HashMap;
@@ -21,8 +33,39 @@ import java.util.concurrent.TimeUnit;
 class NetherAccessListener extends ApplianceListener<NetherAccess> {
     private static final long MESSAGE_COOLDOWN_NANOS = TimeUnit.SECONDS.toNanos(3);
     private final Map<UUID, Long> lastDeniedMessageAt = new HashMap<>();
+    private final Map<UUID, IgnitionAttempt> ignitionAttempts = new HashMap<>();
 
-    @EventHandler(ignoreCancelled = true)
+    @EventHandler(priority = EventPriority.HIGHEST)
+    public void onSpecialItemUse(PlayerInteractEvent event) {
+        if(!NetherPortalIgniter.isSpecial(event.getItem()) || event.getPlayer().getGameMode() == GameMode.CREATIVE) return;
+        event.setCancelled(true);
+        this.ignitionAttempts.remove(event.getPlayer().getUniqueId());
+        if(!this.canSendMessage(event.getPlayer())) return;
+        event.getPlayer().sendMessage(DimensionScheduleText.message(
+            "Das Portalfeuerzeug kann nur im Kreativmodus verwendet werden.", DimensionScheduleText.NETHER_RED
+        ));
+    }
+
+    @EventHandler(priority = EventPriority.MONITOR)
+    public void onIgnitionAttempt(PlayerInteractEvent event) {
+        if(event.getAction() != Action.RIGHT_CLICK_BLOCK) return;
+        UUID playerId = event.getPlayer().getUniqueId();
+        // Record the hand actually used, including ordinary tools. Merely holding the special
+        // item in the other hand must never turn a normal ignition into a marked portal.
+        this.ignitionAttempts.remove(playerId);
+        if(event.useItemInHand() == Event.Result.DENY || event.useInteractedBlock() == Event.Result.DENY) return;
+        if(event.getClickedBlock() == null || event.getHand() == null || event.getItem() == null) return;
+        Material material = event.getItem().getType();
+        if(material != Material.FLINT_AND_STEEL && material != Material.FIRE_CHARGE) return;
+        Block ignition = event.getClickedBlock().getRelative(event.getBlockFace());
+        this.ignitionAttempts.put(playerId, new IgnitionAttempt(
+            ignition.getWorld().getUID(), event.getHand(), Bukkit.getCurrentTick(),
+            new MarkedNetherPortal.Position(ignition.getX(), ignition.getY(), ignition.getZ()),
+            NetherPortalIgniter.isSpecial(event.getItem())
+        ));
+    }
+
+    @EventHandler(priority = EventPriority.HIGHEST, ignoreCancelled = true)
     public void onPortalCreate(PortalCreateEvent event) {
         boolean nether = event.getWorld().getEnvironment() == World.Environment.NETHER;
         boolean blockedCreation = switch(event.getReason()) {
@@ -30,7 +73,30 @@ class NetherAccessListener extends ApplianceListener<NetherAccess> {
             case NETHER_PAIR -> nether;
             default -> false;
         };
-        if(blockedCreation) this.cancelIfClosed(event, event.getEntity());
+        if(!blockedCreation || !this.getAppliance().isClosed()) return;
+        if(event.getReason() == PortalCreateEvent.CreateReason.FIRE && event.getEntity() instanceof Player player) {
+            IgnitionAttempt attempt = this.ignitionAttempts.remove(player.getUniqueId());
+            if(attempt != null && attempt.special() && player.getGameMode() == GameMode.CREATIVE
+                && attempt.tick() == Bukkit.getCurrentTick() && attempt.worldId().equals(event.getWorld().getUID())
+                && event.getBlocks().stream().anyMatch(block -> block.getType() == Material.NETHER_PORTAL
+                    && block.getX() == attempt.ignition().x() && block.getY() == attempt.ignition().y()
+                    && block.getZ() == attempt.ignition().z())) {
+                event.setCancelled(true);
+                this.getAppliance().markedPortals().mark(event, player, attempt.ignition());
+                return;
+            }
+        }
+        this.cancelIfClosed(event, event.getEntity());
+    }
+
+    @EventHandler
+    public void onChunkLoad(ChunkLoadEvent event) {
+        this.getAppliance().markedPortals().onChunkLoad(event.getChunk());
+    }
+
+    @EventHandler
+    public void onWorldLoad(WorldLoadEvent event) {
+        this.getAppliance().markedPortals().onWorldLoad(event.getWorld());
     }
 
     @EventHandler(ignoreCancelled = true)
@@ -46,6 +112,7 @@ class NetherAccessListener extends ApplianceListener<NetherAccess> {
     @EventHandler
     public void onPlayerQuit(PlayerQuitEvent event) {
         this.lastDeniedMessageAt.remove(event.getPlayer().getUniqueId());
+        this.ignitionAttempts.remove(event.getPlayer().getUniqueId());
     }
 
     private static boolean isNetherDestination(@Nullable Location destination) {
@@ -58,11 +125,19 @@ class NetherAccessListener extends ApplianceListener<NetherAccess> {
         event.setCancelled(true);
         if(!(actor instanceof Player player)) return;
 
+        if(this.canSendMessage(player)) this.getAppliance().effects().denyActivation(player);
+    }
+
+    private boolean canSendMessage(Player player) {
         // Cancellation always applies; message and sound share an elapsed-real-time cooldown.
         long now = System.nanoTime();
         Long previousMessageAt = this.lastDeniedMessageAt.get(player.getUniqueId());
-        if(previousMessageAt != null && now - previousMessageAt < MESSAGE_COOLDOWN_NANOS) return;
+        if(previousMessageAt != null && now - previousMessageAt < MESSAGE_COOLDOWN_NANOS) return false;
         this.lastDeniedMessageAt.put(player.getUniqueId(), now);
-        this.getAppliance().effects().denyActivation(player);
+        return true;
+    }
+
+    private record IgnitionAttempt(UUID worldId, EquipmentSlot hand, int tick,
+                                   MarkedNetherPortal.Position ignition, boolean special) {
     }
 }

common/src/main/java/eu/mhsl/craftattack/spawn/common/appliances/tooling/netherAccess/NetherIgniterCommand.java

@@ -0,0 +1,35 @@
+package eu.mhsl.craftattack.spawn.common.appliances.tooling.netherAccess;
+
+import eu.mhsl.craftattack.spawn.common.appliances.tooling.dimensionScheduler.DimensionScheduleText;
+import eu.mhsl.craftattack.spawn.core.appliance.ApplianceCommand;
+import org.bukkit.GameMode;
+import org.bukkit.command.Command;
+import org.bukkit.command.CommandSender;
+import org.bukkit.entity.Player;
+import org.jetbrains.annotations.NotNull;
+
+import java.util.List;
+
+class NetherIgniterCommand extends ApplianceCommand<NetherAccess> {
+    public NetherIgniterCommand() {
+        super("netherIgniter", DimensionScheduleText.message("Fehler: ", DimensionScheduleText.NETHER_RED));
+    }
+
+    @Override
+    protected void execute(@NotNull CommandSender sender, @NotNull Command command, @NotNull String label, @NotNull String[] args) {
+        if(!(sender instanceof Player player)) throw new Error("Dieser Befehl kann nur von Spielern ausgeführt werden.");
+        if(!player.isOp()) throw new Error("Dieser Befehl ist nur für OPs verfügbar.");
+        if(player.getGameMode() != GameMode.CREATIVE) throw new Error("Das Portalfeuerzeug ist nur im Kreativmodus verfügbar.");
+        if(args.length != 0) throw new Error("Verwendung: /netherIgniter");
+        if(player.getInventory().firstEmpty() == -1) throw new Error("Dein Inventar ist voll. Schaffe einen freien Platz für das Portalfeuerzeug.");
+        if(!player.getInventory().addItem(NetherPortalIgniter.create()).isEmpty()) {
+            throw new Error("Das Portalfeuerzeug konnte nicht in dein Inventar gelegt werden.");
+        }
+        player.sendMessage(DimensionScheduleText.message("Du hast ein Portalfeuerzeug erhalten.", DimensionScheduleText.NETHER_RED));
+    }
+
+    @Override
+    public @NotNull List<String> onTabComplete(@NotNull CommandSender sender, @NotNull Command command, @NotNull String label, @NotNull String[] args) {
+        return List.of();
+    }
+}

common/src/main/java/eu/mhsl/craftattack/spawn/common/appliances/tooling/netherAccess/NetherPortalIgniter.java

@@ -0,0 +1,32 @@
+package eu.mhsl.craftattack.spawn.common.appliances.tooling.netherAccess;
+
+import eu.mhsl.craftattack.spawn.common.appliances.tooling.dimensionScheduler.DimensionScheduleText;
+import eu.mhsl.craftattack.spawn.core.Main;
+import eu.mhsl.craftattack.spawn.core.util.inventory.ItemBuilder;
+import org.bukkit.Material;
+import org.bukkit.NamespacedKey;
+import org.bukkit.inventory.ItemStack;
+import org.bukkit.persistence.PersistentDataType;
+import org.jetbrains.annotations.Nullable;
+
+/** The data marker identifies the item independently of its displayed name. */
+final class NetherPortalIgniter {
+    private static final NamespacedKey KEY = new NamespacedKey(Main.instance(), "nether_portal_igniter");
+
+    private NetherPortalIgniter() {
+    }
+
+    static ItemStack create() {
+        return ItemBuilder.of(Material.FLINT_AND_STEEL)
+            .displayName(DimensionScheduleText.message("Portalfeuerzeug", DimensionScheduleText.NETHER_RED))
+            .lore("Nur im Kreativmodus verwendbar. Bei geschlossenem Nether werden gültige Portale zur späteren Öffnung vorgemerkt. Bei offenem Nether funktioniert es wie ein normales Feuerzeug.")
+            .glint()
+            .withMeta(meta -> meta.getPersistentDataContainer().set(KEY, PersistentDataType.BOOLEAN, true))
+            .build();
+    }
+
+    static boolean isSpecial(@Nullable ItemStack item) {
+        return item != null && item.getType() == Material.FLINT_AND_STEEL && item.hasItemMeta()
+            && Boolean.TRUE.equals(item.getItemMeta().getPersistentDataContainer().get(KEY, PersistentDataType.BOOLEAN));
+    }
+}

common/src/main/java/eu/mhsl/craftattack/spawn/common/appliances/tooling/netherAccess/PortalMarkStore.java

@@ -0,0 +1,237 @@
+package eu.mhsl.craftattack.spawn.common.appliances.tooling.netherAccess;
+
+import org.bukkit.Axis;
+import org.bukkit.configuration.InvalidConfigurationException;
+import org.bukkit.configuration.file.YamlConfiguration;
+
+import java.io.IOException;
+import java.io.Reader;
+import java.nio.charset.StandardCharsets;
+import java.nio.file.AtomicMoveNotSupportedException;
+import java.nio.file.Files;
+import java.nio.file.Path;
+import java.nio.file.StandardCopyOption;
+import java.util.LinkedHashMap;
+import java.util.List;
+import java.util.Map;
+import java.util.Objects;
+import java.util.Set;
+import java.util.UUID;
+import java.util.concurrent.Callable;
+import java.util.concurrent.CompletableFuture;
+import java.util.concurrent.ConcurrentHashMap;
+import java.util.concurrent.ExecutorService;
+import java.util.concurrent.Executors;
+import java.util.concurrent.RejectedExecutionException;
+import java.util.concurrent.TimeUnit;
+import java.util.concurrent.locks.ReentrantLock;
+import java.util.logging.Logger;
+
+/** Serial file ownership keeps failed writes out of both the committed map and later snapshots. */
+final class PortalMarkStore implements AutoCloseable {
+    private static final int FORMAT_VERSION = 1;
+    private static final Logger LOGGER = Logger.getLogger(PortalMarkStore.class.getName());
+    // Kept across store instances: timed-out old IO must finish before a restarted appliance can load this path.
+    private static final ConcurrentHashMap<Path, ReentrantLock> PATH_LOCKS = new ConcurrentHashMap<>();
+
+    private final Path path;
+    private final ReentrantLock pathLock;
+    private final ExecutorService worker = Executors.newSingleThreadExecutor(task -> {
+        Thread thread = new Thread(task, "Spawn-PortalMarks");
+        thread.setDaemon(true);
+        return thread;
+    });
+    private final Set<CompletableFuture<?>> pending = ConcurrentHashMap.newKeySet();
+
+    // Only the IO worker accesses the following state.
+    private Map<String, MarkedNetherPortal> committed = new LinkedHashMap<>();
+    private boolean loaded;
+
+    PortalMarkStore(Path path) {
+        this.path = Objects.requireNonNull(path, "path").toAbsolutePath().normalize();
+        this.pathLock = PATH_LOCKS.computeIfAbsent(this.path, ignored -> new ReentrantLock());
+    }
+
+    CompletableFuture<List<MarkedNetherPortal>> load() {
+        return this.submit(() -> {
+            if(this.loaded) return List.copyOf(this.committed.values());
+            Map<String, MarkedNetherPortal> entries = new LinkedHashMap<>();
+            if(!Files.notExists(this.path)) {
+                YamlConfiguration yaml = new YamlConfiguration();
+                try(Reader reader = Files.newBufferedReader(this.path, StandardCharsets.UTF_8)) {
+                    yaml.load(reader);
+                } catch(InvalidConfigurationException exception) {
+                    throw new IOException("Invalid portal marking YAML: " + this.path, exception);
+                }
+                Object version = yaml.get("version");
+                if(!(version instanceof Integer number) || number != FORMAT_VERSION) {
+                    throw new IOException("Unsupported or missing portal marking version: " + this.path);
+                }
+                if(!(yaml.get("portals") instanceof List<?> portals)) {
+                    throw new IOException("Missing portal marking list: " + this.path);
+                }
+                for(int index = 0; index < portals.size(); index++) {
+                    MarkedNetherPortal portal;
+                    try {
+                        portal = readPortal(portals.get(index));
+                    } catch(RuntimeException exception) {
+                        throw new IOException("Invalid portal marking at index " + index + ": " + this.path, exception);
+                    }
+                    if(entries.putIfAbsent(portal.id(), portal) != null) {
+                        throw new IOException("Duplicate portal marking: " + portal.id());
+                    }
+                }
+            }
+            this.committed = entries;
+            this.loaded = true;
+            return List.copyOf(entries.values());
+        });
+    }
+
+    CompletableFuture<MarkedNetherPortal> putIfAbsent(MarkedNetherPortal portal) {
+        return this.submit(() -> {
+            this.requireLoaded();
+            Objects.requireNonNull(portal, "portal");
+            MarkedNetherPortal existing = this.committed.get(portal.id());
+            if(existing != null) return existing;
+            Map<String, MarkedNetherPortal> snapshot = new LinkedHashMap<>(this.committed);
+            snapshot.put(portal.id(), portal);
+            this.save(snapshot);
+            this.committed = snapshot;
+            return portal;
+        });
+    }
+
+    CompletableFuture<Void> remove(String id) {
+        return this.submit(() -> {
+            this.requireLoaded();
+            if(!this.committed.containsKey(id)) return null;
+            Map<String, MarkedNetherPortal> snapshot = new LinkedHashMap<>(this.committed);
+            snapshot.remove(id);
+            this.save(snapshot);
+            this.committed = snapshot;
+            return null;
+        });
+    }
+
+    private void requireLoaded() throws IOException {
+        if(!this.loaded) throw new IOException("Portal markings have not been loaded successfully: " + this.path);
+    }
+
+    private void save(Map<String, MarkedNetherPortal> snapshot) throws IOException {
+        YamlConfiguration yaml = new YamlConfiguration();
+        yaml.set("version", FORMAT_VERSION);
+        yaml.set("portals", snapshot.values().stream().map(PortalMarkStore::writePortal).toList());
+
+        Path directory = this.path.getParent();
+        Files.createDirectories(directory);
+        Path temporary = Files.createTempFile(directory, this.path.getFileName() + ".", ".tmp");
+        try {
+            Files.writeString(temporary, yaml.saveToString(), StandardCharsets.UTF_8);
+            try {
+                Files.move(temporary, this.path, StandardCopyOption.ATOMIC_MOVE, StandardCopyOption.REPLACE_EXISTING);
+            } catch(AtomicMoveNotSupportedException exception) {
+                Files.move(temporary, this.path, StandardCopyOption.REPLACE_EXISTING);
+            }
+        } catch(IOException | RuntimeException failure) {
+            try {
+                Files.deleteIfExists(temporary);
+            } catch(IOException cleanupFailure) {
+                failure.addSuppressed(cleanupFailure);
+            }
+            throw failure;
+        }
+    }
+
+    private static Map<String, Object> writePortal(MarkedNetherPortal portal) {
+        Map<String, Object> values = new LinkedHashMap<>();
+        values.put("world", portal.worldId().toString());
+        values.put("creator", portal.creatorId().toString());
+        values.put("origin", writePosition(portal.origin()));
+        values.put("width", portal.width());
+        values.put("height", portal.height());
+        values.put("axis", portal.axis().name());
+        values.put("ignition", writePosition(portal.ignition()));
+        values.put("frame", portal.frame().stream().map(PortalMarkStore::writePosition).toList());
+        return values;
+    }
+
+    private static List<Integer> writePosition(MarkedNetherPortal.Position position) {
+        return List.of(position.x(), position.y(), position.z());
+    }
+
+    private static MarkedNetherPortal readPortal(Object value) {
+        if(!(value instanceof Map<?, ?> values)) throw new IllegalArgumentException("Expected a portal mapping");
+        if(!(values.get("frame") instanceof List<?> frame)) throw new IllegalArgumentException("Missing portal frame");
+        return new MarkedNetherPortal(
+            UUID.fromString(string(values, "world")),
+            UUID.fromString(string(values, "creator")),
+            readPosition(values.get("origin")),
+            integer(values.get("width")),
+            integer(values.get("height")),
+            Axis.valueOf(string(values, "axis")),
+            readPosition(values.get("ignition")),
+            frame.stream().map(PortalMarkStore::readPosition).toList()
+        );
+    }
+
+    private static String string(Map<?, ?> values, String key) {
+        if(values.get(key) instanceof String value) return value;
+        throw new IllegalArgumentException("Missing portal string: " + key);
+    }
+
+    private static MarkedNetherPortal.Position readPosition(Object value) {
+        if(!(value instanceof List<?> coordinates) || coordinates.size() != 3) {
+            throw new IllegalArgumentException("Expected three integer coordinates");
+        }
+        return new MarkedNetherPortal.Position(
+            integer(coordinates.get(0)), integer(coordinates.get(1)), integer(coordinates.get(2))
+        );
+    }
+
+    private static int integer(Object value) {
+        if(value instanceof Integer number) return number;
+        throw new IllegalArgumentException("Expected an integer");
+    }
+
+    private <T> CompletableFuture<T> submit(Callable<T> operation) {
+        CompletableFuture<T> result = new CompletableFuture<>();
+        this.pending.add(result);
+        result.whenComplete((value, failure) -> this.pending.remove(result));
+        try {
+            this.worker.execute(() -> {
+                if(result.isDone()) return;
+                try {
+                    T value;
+                    this.pathLock.lockInterruptibly();
+                    try {
+                        if(result.isDone()) return;
+                        value = operation.call();
+                    } finally {
+                        this.pathLock.unlock();
+                    }
+                    result.complete(value);
+                } catch(Throwable failure) {
+                    result.completeExceptionally(failure);
+                }
+            });
+        } catch(RejectedExecutionException failure) {
+            result.completeExceptionally(failure);
+        }
+        return result;
+    }
+
+    @Override
+    public void close() {
+        this.worker.shutdown();
+        try {
+            if(this.worker.awaitTermination(5, TimeUnit.SECONDS)) return;
+            LOGGER.warning("Timed out while saving portal markings: " + this.path);
+        } catch(InterruptedException exception) {
+            Thread.currentThread().interrupt();
+        }
+        this.worker.shutdownNow();
+        IOException failure = new IOException("Portal marking storage stopped before queued operations completed");
+        this.pending.forEach(future -> future.completeExceptionally(failure));
+    }
+}