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/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()));