chore: Halkyon Learn moves into apps/learn

Its server, client, app and curator, and the tools that write its
assets (maps, Anki fixtures, icons), as they are; paths are fixed in
the commits that follow.

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/learn/halkyon_learn_flutter/test/feature/settings/data/settings_repository_test.dart

@@ -0,0 +1,119 @@
+import 'package:flutter_test/flutter_test.dart';
+import 'package:halkyon_learn_flutter/src/core/content/payload.dart';
+import 'package:halkyon_learn_flutter/src/feature/settings/data/settings_payload.dart';
+import 'package:halkyon_learn_flutter/src/feature/settings/data/settings_repository.dart';
+import 'package:halkyon_learn_flutter/src/feature/settings/model/person_settings.dart';
+
+import '../../../support/fake_settings_gateway.dart';
+
+void main() {
+  late FakeSettingsGateway gateway;
+  late SettingsRepository repository;
+
+  setUp(() {
+    gateway = FakeSettingsGateway();
+    repository = SettingsRepository(gateway);
+  });
+  tearDown(() => repository.dispose());
+
+  const verb = (id: 'tag-1', name: 'verb');
+  const noun = (id: 'tag-2', name: 'noun');
+
+  test('a person without settings has the defaults, and nothing is '
+      'stored for reading them', () async {
+    expect(await repository.read(), const PersonSettings());
+    expect(gateway.writes, isEmpty);
+  });
+
+  test('stores a change in one row, made by the first change', () async {
+    await repository.update((s) => s.withStudyTags('japanese', [verb]));
+    await repository.update((s) => s.withStudyTags('german', [noun]));
+
+    expect(
+      await repository.read(),
+      const PersonSettings(
+        studyTags: {
+          'japanese': [verb],
+          'german': [noun],
+        },
+      ),
+    );
+    expect({for (final (id, _) in gateway.writes) id}, hasLength(1));
+  });
+
+  test('two rows from two devices fold into the one with the smaller id, '
+      "with the other's settings added where it has none", () async {
+    await repository.update((s) => s.withStudyTags('japanese', [verb]));
+    gateway.receiveRemoteRow(
+      'settings-000',
+      encodeSettingsPayload(
+        const PersonSettings(
+          studyTags: {
+            'japanese': [noun],
+            'german': [noun],
+          },
+        ),
+      ),
+    );
+
+    final settings = await repository.read();
+
+    expect(
+      settings,
+      const PersonSettings(
+        studyTags: {
+          'japanese': [noun],
+          'german': [noun],
+        },
+      ),
+    );
+    expect(gateway.deleted, ['settings-001']);
+    // Blanked before deleted.
+    expect(gateway.writes.last, ('settings-001', redactedPayload));
+  });
+
+  test('leaves settings of a newer app version alone, and keeps its own '
+      'beside them', () async {
+    gateway.receiveRemoteRow(
+      'settings-000',
+      encodePayload(settingsSchemaVersion + 1, {'studyTags': {}}),
+    );
+
+    await repository.update((s) => s.withStudyTags('japanese', [verb]));
+
+    expect(
+      (await repository.read()).studyTagsOf('japanese'),
+      [verb],
+    );
+    expect(gateway.deleted, isEmpty);
+    expect(gateway.writes.single.$1, isNot('settings-000'));
+  });
+
+  test('follows changes on this device and from others', () async {
+    final watched = repository.watch();
+    final expectation = expectLater(
+      watched.map((s) => s.studyTags.keys.toList()),
+      emitsInOrder([
+        <String>[],
+        ['japanese'],
+        ['japanese', 'german'],
+      ]),
+    );
+    await Future<void>.delayed(Duration.zero);
+
+    await repository.update((s) => s.withStudyTags('japanese', [verb]));
+    final id = gateway.writes.single.$1;
+    gateway.receiveRemoteRow(
+      id,
+      encodeSettingsPayload(
+        const PersonSettings(
+          studyTags: {
+            'japanese': [verb],
+            'german': [noun],
+          },
+        ),
+      ),
+    );
+    await expectation;
+  });
+}