simplify: dedupe boilerplate and remove dead code across all layers

lib/view/pages/timetable/data/timetable_appointment_factory.dart

@@ -8,6 +8,7 @@ import '../../../../api/mhsl/custom_timetable_event/custom_timetable_event.dart'
 import '../../../../storage/timetable_settings.dart';
 import 'arbitrary_appointment.dart';
 import 'lesson_color.dart';
+import 'lesson_merger.dart';
 import 'lesson_status.dart';
 import 'lesson_type_label.dart';
 import 'rrule_with_exceptions.dart';
@@ -33,7 +34,7 @@ class TimetableAppointmentFactory {
 
   List<Appointment> build() {
     final source = settings.connectDoubleLessons
-        ? _mergeAdjacentLessons(lessons)
+        ? LessonMerger.merge(lessons)
         : lessons;
     return [
       ...source.map(_lessonToAppointment),
@@ -267,68 +268,4 @@ class TimetableAppointmentFactory {
         .trim();
     return cleaned.isEmpty ? null : cleaned;
   }
-
-  // Pure: builds a new list, does not mutate inputs. The previous version
-  // mutated `previous.endTime` in place which caused merged blocks to grow
-  // further on subsequent rebuilds when the same lesson objects were observed
-  // again by the next merge pass.
-  static List<McTimetableEntry> _mergeAdjacentLessons(
-    List<McTimetableEntry> input, {
-    Duration maxGap = const Duration(minutes: 5),
-  }) {
-    if (input.isEmpty) return const [];
-
-    final sorted = [...input]
-      ..sort((a, b) => a.startDateTime.compareTo(b.startDateTime));
-
-    final merged = <McTimetableEntry>[];
-    for (final current in sorted) {
-      if (merged.isNotEmpty && _canMerge(merged.last, current, maxGap)) {
-        final prev = merged.removeLast();
-        merged.add(_extendedEnd(prev, current.endTime));
-      } else {
-        merged.add(current);
-      }
-    }
-    return merged;
-  }
-
-  static McTimetableEntry _extendedEnd(
-    McTimetableEntry source,
-    DateTime newEndTime,
-  ) => McTimetableEntry(
-    id: source.id,
-    date: source.date,
-    startTime: source.startTime,
-    endTime: newEndTime,
-    subjects: source.subjects,
-    teachers: source.teachers,
-    rooms: source.rooms,
-    classNames: source.classNames,
-    lessonType: source.lessonType,
-    status: source.status,
-    substitutionText: source.substitutionText,
-    lessonText: source.lessonText,
-    infoText: source.infoText,
-  );
-
-  static bool _canMerge(
-    McTimetableEntry a,
-    McTimetableEntry b,
-    Duration maxGap,
-  ) {
-    if (a.subjects.firstOrNull != b.subjects.firstOrNull) return false;
-    if (a.rooms.firstOrNull != b.rooms.firstOrNull) return false;
-    if (a.teachers.firstOrNull?.shortName !=
-        b.teachers.firstOrNull?.shortName) {
-      return false;
-    }
-    if (a.status != b.status) return false;
-
-    // Merge only sequential lessons (b starts at or after a ends, within the
-    // tolerance). Without the lower bound, identical-metadata lessons that
-    // overlap in time would silently collapse into one.
-    final gap = b.startDateTime.difference(a.endDateTime);
-    return !gap.isNegative && gap <= maxGap;
-  }
 }