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/services/audio_service.dart

@@ -0,0 +1,42 @@
+import 'package:audioplayers/audioplayers.dart';
+
+enum Sfx { place, clear, combo, win, lose }
+
+/// Fire-and-forget SFX. A small player pool avoids cutting sounds off when
+/// effects overlap (place + clear on the same move).
+class AudioService {
+  AudioService({this.enabled = true});
+
+  bool enabled;
+
+  static const _files = {
+    Sfx.place: 'audio/place.wav',
+    Sfx.clear: 'audio/clear.wav',
+    Sfx.combo: 'audio/combo.wav',
+    Sfx.win: 'audio/win.wav',
+    Sfx.lose: 'audio/lose.wav',
+  };
+
+  final _players = <AudioPlayer>[
+    for (var i = 0; i < 3; i++) AudioPlayer()..setReleaseMode(ReleaseMode.stop),
+  ];
+  var _next = 0;
+
+  Future<void> play(Sfx sfx) async {
+    if (!enabled) return;
+    final player = _players[_next];
+    _next = (_next + 1) % _players.length;
+    try {
+      await player.stop();
+      await player.play(AssetSource(_files[sfx]!));
+    } catch (_) {
+      // Audio must never break gameplay.
+    }
+  }
+
+  void dispose() {
+    for (final p in _players) {
+      p.dispose();
+    }
+  }
+}

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;
+});

lib/ui/screens/game_screen.dart

@@ -4,6 +4,7 @@ import 'package:flutter_riverpod/flutter_riverpod.dart';
 import '../../game/engine/game_engine.dart';
 import '../../game/models/stage.dart';
 import '../../l10n/gen/app_localizations.dart';
+import '../../services/audio_service.dart';
 import '../../state/game_session_notifier.dart';
 import '../../state/providers.dart';
 import '../theme/palette.dart';
@@ -124,8 +125,26 @@ class _GameScreenState extends ConsumerState<GameScreen> {
     }
   }
 
+  void _playSfx(GameViewState? prev, GameViewState? next) {
+    if (next == null) return;
+    final audio = ref.read(audioServiceProvider);
+    if (prev?.fxTick != next.fxTick && next.lastPlacement != null) {
+      final placement = next.lastPlacement!;
+      if (placement.linesCleared > 0) {
+        audio.play(placement.comboStreak >= 2 ? Sfx.combo : Sfx.clear);
+      } else {
+        audio.play(Sfx.place);
+      }
+    }
+    if (prev?.phase != next.phase) {
+      if (next.phase == GamePhase.won) audio.play(Sfx.win);
+      if (next.phase == GamePhase.lost) audio.play(Sfx.lose);
+    }
+  }
+
   @override
   Widget build(BuildContext context) {
+    ref.listen<GameViewState?>(gameSessionProvider, _playSfx);
     final view = ref.watch(gameSessionProvider);
     if (view == null) {
       return const Scaffold(body: Center(child: CircularProgressIndicator()));