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>

lib/state/providers.dart

@@ -1,8 +1,15 @@
 import 'package:flutter_riverpod/flutter_riverpod.dart';
 
+import '../services/audio_service.dart';
 import 'game_session_notifier.dart';
 
 final gameSessionProvider =
     NotifierProvider<GameSessionNotifier, GameViewState?>(
   GameSessionNotifier.new,
 );
+
+final audioServiceProvider = Provider<AudioService>((ref) {
+  final service = AudioService();
+  ref.onDispose(service.dispose);
+  return service;
+});