feat: notes in studysets, synced and merged as cards are, with tags

Server: three synced tables, only additions (migration
20261002191122122-notes): `note` (its studyset, a fractional position),
`note_revision` (append-only, as a card's: parents, author, report,
the payload {schemaVersion, title, body}) and `note_tag` (one row per
assignment, never updated, as `card_tag`). Each indexes `spaceId`, and
everything goes with its space, studyset, note or tag. A migration of
synced tables locks app builds from before it out of syncing (ADR 0002,
rule 3): cheap now, before the first release.

App, `feature/note`:
- `NoteContent` is a title and rich text, merged apart (ADR 0003): a
  title changed on one device and the body on another both survive, a
  body changed on two is a conflict.
- `NoteRepository` creates a note at the end of its studyset, saves an
  edit on the revisions it started from (`saveEdit`, as the card
  composer now does), says when a note was last edited (its newest
  head's UUID v7), counts notes per studyset, and deletes a note's
  revisions blanked first (ADR 0002, rule 2).
- Notes share the person's tags: `setTagsOfNote`, and folding tags of
  one name or undoing a tag's deletion covers notes too.
- The delayed deletion with its Undo moves to `core/async`
  (`DelayedDeletions`): `StudysetDeletions` builds on it and now takes
  a studyset's notes too; `NoteDeletions` deletes a single note.

Tests: content, merge, payload, repository, deletions, tags, the device
migration guard, and sync tests against the server (four).

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>
Signed-off-by: Tizian 「ティツィアーン」 <raisondetredev@pm.me>

halkyon_learn_flutter/test/sync/support/sync_devices.dart

@@ -9,6 +9,9 @@ import 'package:halkyon_learn_flutter/src/core/sync/sync_service.dart';
 import 'package:halkyon_learn_flutter/src/feature/card/data/card_repository.dart';
 import 'package:halkyon_learn_flutter/src/feature/card/data/sync_card_gateway.dart';
 import 'package:halkyon_learn_flutter/src/feature/card/model/card.dart';
+import 'package:halkyon_learn_flutter/src/feature/note/data/note_repository.dart';
+import 'package:halkyon_learn_flutter/src/feature/note/data/sync_note_gateway.dart';
+import 'package:halkyon_learn_flutter/src/feature/note/model/note.dart';
 import 'package:halkyon_learn_flutter/src/feature/settings/data/settings_repository.dart';
 import 'package:halkyon_learn_flutter/src/feature/settings/data/sync_settings_gateway.dart';
 import 'package:halkyon_learn_flutter/src/feature/study/data/review_repository.dart';
@@ -121,6 +124,9 @@ final class SyncDevice(
   /// The app's cards on this device's database.
   final CardRepository cards,
 
+  /// The app's notes on this device's database.
+  final NoteRepository notes,
+
   /// The app's reviews on this device's database.
   final ReviewRepository reviews,
 
@@ -154,16 +160,21 @@ final class SyncDevice(
     final cards = CardRepository(
       SyncCardGateway(db, syncRequests, person.authUserId.uuid),
     );
+    final notes = NoteRepository(
+      SyncNoteGateway(db, syncRequests, person.authUserId.uuid),
+    );
     return SyncDevice(
       client,
       db,
       syncRequests,
       StudysetRepository(SyncStudysetGateway(db, syncRequests)),
       cards,
+      notes,
       ReviewRepository(SyncReviewGateway(db, syncRequests)),
       TagRepository(
         SyncTagGateway(db, syncRequests),
         cardChanges: cards.changes,
+        noteChanges: notes.changes,
       ),
       SettingsRepository(SyncSettingsGateway(db, syncRequests)),
       AssetRepository(
@@ -197,6 +208,16 @@ final class SyncDevice(
     return null;
   }
 
+  /// The note with [id] in [studysetId] on this device, or null.
+  Future<Note?> note(String studysetId, String id) async {
+    for (final note in await notes.notesOf(studysetId)) {
+      if (note.id == id) {
+        return note;
+      }
+    }
+    return null;
+  }
+
   /// Closes everything the device opened.
   Future<void> close() async {
     await studysets.dispose();
@@ -204,6 +225,7 @@ final class SyncDevice(
     await settings.dispose();
     await reviews.dispose();
     await cards.dispose();
+    await notes.dispose();
     await assets.dispose();
     await _syncRequests.dispose();
     await _db.close();