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_client/lib/migrations/migration_registry.dart

@@ -8,6 +8,7 @@ part '20260930094729362-studysets/migration.dart';
 part '20260930122235111-cards/migration.dart';
 part '20260930145136285-reviews/migration.dart';
 part '20261001064926911-tree-tags-assets/migration.dart';
+part '20261002191122122-notes/migration.dart';
 
 /// Migration registry for the client-side database.
 class MigrationRegistry {
@@ -18,6 +19,7 @@ class MigrationRegistry {
     _Migration20260930122235111_cards(),
     _Migration20260930145136285_reviews(),
     _Migration20261001064926911_tree_tags_assets(),
+    _Migration20261002191122122_notes(),
   ];
 
   /// List of all client-side database migration versions.