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();
+    }
+  }
+}