Add synthesized SFX and audio wiring

Pure-Dart WAV synthesizer (tool/gen_sfx.dart) generates place/clear/
combo/win/lose effects; AudioService player pool fires on placement,
line clears, combo streaks, and phase transitions.

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

test/ui/game_screen_golden_test.dart

@@ -0,0 +1,68 @@
+import 'package:block_seasons/core/rng.dart';
+import 'package:block_seasons/game/engine/piece_generator.dart';
+import 'package:block_seasons/l10n/gen/app_localizations.dart';
+import 'package:block_seasons/game/models/stage.dart';
+import 'package:block_seasons/state/providers.dart';
+import 'package:block_seasons/ui/screens/game_screen.dart';
+import 'package:flutter/material.dart';
+import 'package:flutter_riverpod/flutter_riverpod.dart';
+import 'package:flutter_test/flutter_test.dart';
+
+final _stage = StageConfig.fromJson({
+  'id': 'golden_stage',
+  'seed': 777,
+  'moveLimit': 22,
+  'preset': [
+    {'x': 2, 'y': 2, 't': 'gem'},
+    {'x': 5, 'y': 2, 't': 'gem'},
+    {'x': 2, 'y': 5, 't': 'gem'},
+    {'x': 3, 'y': 7, 't': 'filled', 'c': 1},
+    {'x': 4, 'y': 7, 't': 'filled', 'c': 2},
+    {'x': 5, 'y': 7, 't': 'filled', 'c': 3},
+    {'x': 1, 'y': 6, 't': 'filled', 'c': 4},
+  ],
+  'objectives': [
+    {'type': 'clearGems', 'count': 3},
+  ],
+  'stars': {
+    'two': {'movesLeft': 5},
+    'three': {'movesLeft': 10},
+  },
+  'generatorProfile': 'mid',
+});
+
+void main() {
+  testWidgets('game screen golden snapshot', (tester) async {
+    await tester.binding.setSurfaceSize(const Size(390, 844)); // iPhone-ish
+    final container = ProviderContainer();
+    addTearDown(container.dispose);
+    container
+        .read(gameSessionProvider.notifier)
+        .startStage(_stage, generator: PieceGenerator(SeededRng(7)));
+
+    await tester.pumpWidget(
+      UncontrolledProviderScope(
+        container: container,
+        child: MaterialApp(
+          debugShowCheckedModeBanner: false,
+          localizationsDelegates: AppLocalizations.localizationsDelegates,
+          supportedLocales: AppLocalizations.supportedLocales,
+          theme: ThemeData(
+            colorScheme: ColorScheme.fromSeed(
+              seedColor: const Color(0xFF5B7FFF),
+              brightness: Brightness.dark,
+            ),
+            useMaterial3: true,
+          ),
+          home: const GameScreen(),
+        ),
+      ),
+    );
+    await tester.pumpAndSettle();
+
+    await expectLater(
+      find.byType(GameScreen),
+      matchesGoldenFile('goldens/game_screen.png'),
+    );
+  });
+}