feat(todo): the person's projects, sealed and synced, the Inbox made once (ADR 0033, D5, D6)

TodoPerson opens the person's own sync database in their folder,
halkyon_app's SyncService behind AppCompatibility, their key (TodoKeys,
Train's copied with Todo's own key IDs) and their projects, and closes
them in that order, the key last. A project is sealed for its row; a
readable, foreign or moved row shows locked, and a device without the
account's keys reads and writes nothing. The Inbox is the row under an
ID keyed by the account, so every device makes the same one and the
instance can't tell which it is; it can't be deleted. Settings shows
Sync's card first and the cog its dot while sync fails, as Learn's.

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

apps/todo/halkyon_todo_flutter/test/support/memory_content_key_gateway.dart

@@ -0,0 +1,19 @@
+import 'dart:typed_data';
+
+import 'package:halkyon_todo_flutter/src/core/privacy/content_key_gateway.dart';
+
+/// [ContentKeyGateway] in memory, which tests share between "devices" as the
+/// sync engine would, and change as an instance could.
+class MemoryContentKeyGateway() implements ContentKeyGateway {
+  /// The rows, as the instance holds them.
+  final rows = <ContentKeyRow>[];
+
+  @override
+  Future<List<ContentKeyRow>> all() async => List.of(rows);
+
+  @override
+  Future<void> insert({
+    required String dataKeyId,
+    required Uint8List sealedKey,
+  }) async => rows.add((dataKeyId: dataKeyId, sealedKey: sealedKey));
+}

apps/todo/halkyon_todo_flutter/test/support/memory_project_gateway.dart

@@ -0,0 +1,36 @@
+import 'dart:async';
+
+import 'package:halkyon_app/halkyon_app.dart';
+import 'package:halkyon_todo_flutter/src/feature/project/data/project_gateway.dart';
+
+/// [ProjectGateway] in memory, whose rows a test changes as an instance could.
+class MemoryProjectGateway() implements ProjectGateway {
+  final rows = <String, Envelope>{};
+  final _remote = StreamController<void>.broadcast();
+
+  /// As a sync bringing others' changes.
+  void bringRemoteChanges() => _remote.add(null);
+
+  @override
+  Future<List<ProjectRow>> projects() async => [
+    for (final MapEntry(:key, :value) in rows.entries)
+      (id: key, envelope: value),
+  ];
+
+  @override
+  Future<void> insert(ProjectRow project) async =>
+      rows[project.id] = project.envelope;
+
+  @override
+  Future<void> update(String id, Envelope envelope) async {
+    if (rows.containsKey(id)) {
+      rows[id] = envelope;
+    }
+  }
+
+  @override
+  Future<void> delete(String id) async => rows.remove(id);
+
+  @override
+  Stream<void> get remoteChanges => _remote.stream;
+}