feat: content repository merges cached seasons with bundled fallback

Extends ContentRepository with optional cacheDir: cached packs in
<cacheDir>/seasons/*/pack.json merge with bundled ones (cached wins
for same id), corrupt/future-schema packs silently ignored, refresh()
fires ContentDownloader.sync() once per session. main() wires the real
cache+downloader instance; default constructor stays bundled-only for tests.

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>

test/data/content_repository_test.dart

@@ -1,4 +1,8 @@
+import 'dart:convert';
+import 'dart:io';
+
 import 'package:block_seasons/data/content_repository.dart';
+import 'package:block_seasons/game/models/season.dart';
 import 'package:flutter_test/flutter_test.dart';
 
 void main() {
@@ -25,4 +29,75 @@ void main() {
     final seasons = await repo.availableSeasons();
     expect(seasons.map((s) => s.seasonId), contains('season_001'));
   });
+
+  group('cache merge and fallback', () {
+    late Directory tmp;
+
+    setUp(() async {
+      tmp = await Directory.systemTemp.createTemp('bs_repo_');
+    });
+
+    tearDown(() async {
+      if (await tmp.exists()) await tmp.delete(recursive: true);
+    });
+
+    Map<String, dynamic> packJson(String id) => {
+          'schemaVersion': 1,
+          'seasonId': id,
+          'version': 1,
+          'title': {'en': id},
+          'theme': const SeasonTheme().toJson(),
+          'stages': [
+            {
+              'id': '${id}_001',
+              'seed': 1,
+              'moveLimit': 10,
+              'preset': [],
+              'objectives': [
+                {'type': 'reachScore', 'target': 100}
+              ],
+              'stars': {
+                'two': {'movesLeft': 2},
+                'three': {'movesLeft': 4}
+              },
+              'generatorProfile': 'mid',
+            }
+          ],
+        };
+
+    Future<void> putCachedSeason(String id) async {
+      final dir = Directory('${tmp.path}/seasons/$id');
+      await dir.create(recursive: true);
+      await File('${dir.path}/pack.json')
+          .writeAsString(jsonEncode(packJson(id)));
+    }
+
+    test('merges cached seasons after bundled ones, sorted by id', () async {
+      TestWidgetsFlutterBinding.ensureInitialized();
+      await putCachedSeason('season_002');
+      final repo = ContentRepository(cacheDir: tmp);
+      final seasons = await repo.availableSeasons();
+      expect(seasons.map((s) => s.seasonId).toList(),
+          ['season_001', 'season_002']);
+    });
+
+    test('cached copy of a bundled season wins over the bundle', () async {
+      TestWidgetsFlutterBinding.ensureInitialized();
+      await putCachedSeason('season_001');
+      final repo = ContentRepository(cacheDir: tmp);
+      final seasons = await repo.availableSeasons();
+      // Cached fake has exactly 1 stage; the bundled real one has 60.
+      expect(seasons.single.stages, hasLength(1));
+    });
+
+    test('corrupt cached pack falls back to bundle silently', () async {
+      TestWidgetsFlutterBinding.ensureInitialized();
+      final dir = Directory('${tmp.path}/seasons/season_001');
+      await dir.create(recursive: true);
+      await File('${dir.path}/pack.json').writeAsString('{not json');
+      final repo = ContentRepository(cacheDir: tmp);
+      final seasons = await repo.availableSeasons();
+      expect(seasons.single.stages, hasLength(60));
+    });
+  });
 }