Sounds: synthesized sound set and a debug sound board

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

apps/desktop/lib/src/sfx/synth_sfx.dart

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+import 'dart:async';
+import 'dart:io';
+
+import 'package:flutter/foundation.dart';
+import 'package:media_kit/media_kit.dart';
+import 'package:path_provider/path_provider.dart';
+
+import 'sfx.dart';
+import 'sound_bank.dart';
+import 'synth.dart';
+
+/// Plays [SoundSpec]s: each is rendered once to a WAV in the temp dir and
+/// played on a small pool of audio-only mpv players (so two sounds can
+/// overlap). [volume] is 0..1, applied on a square curve (ears are log).
+class SynthSfx implements SfxService {
+  double volume;
+  SynthSfx({this.volume = 0.6});
+
+  static const _poolSize = 3;
+  final List<Player> _pool = [];
+  var _next = 0;
+  final Map<String, Future<String>> _files = {};
+  Directory? _dir;
+
+  @override
+  Future<void> play(SfxCue cue) =>
+      playSpec('${cue.kind.name}-${cue.readyIndex}', soundFor(cue.kind, readyIndex: cue.readyIndex));
+
+  /// Plays [spec], cached under [key] (same key = same sound).
+  Future<void> playSpec(String key, SoundSpec spec) async {
+    if (volume <= 0) return;
+    try {
+      final path = await (_files[key] ??= _write(key, spec));
+      final player = await _player();
+      await player.setVolume(100 * volume * volume);
+      await player.open(Media(path));
+    } catch (e) {
+      debugPrint('sfx $key: $e');
+    }
+  }
+
+  Future<String> _write(String key, SoundSpec spec) async {
+    _dir ??= await Directory('${(await getTemporaryDirectory()).path}/watch_together_sfx').create(recursive: true);
+    final f = File('${_dir!.path}/$key.wav');
+    await f.writeAsBytes(wav(render(spec)));
+    return f.path;
+  }
+
+  Future<Player> _player() async {
+    if (_pool.length < _poolSize) {
+      final p = Player(
+        configuration: const PlayerConfiguration(vo: 'null', osc: false, libass: false, title: 'sfx'),
+      );
+      await (p.platform as NativePlayer).setProperty('audio-display', 'no');
+      _pool.add(p);
+      return p;
+    }
+    return _pool[_next++ % _poolSize];
+  }
+
+  /// Drops the cached renders (after editing a spec with hot reload).
+  void forget() => _files.clear();
+
+  Future<void> dispose() async {
+    for (final p in _pool) {
+      await p.dispose();
+    }
+    _pool.clear();
+  }
+}