Sounds play on flutter_soloud instead of three mpv players

SoundEngine (platform/audio) opens the device once (2 s after start,
or on the first sound), loads each synthesized sound into memory (no
temp files), plays overlapping voices and is disposed on window close,
before leaving the server. The SfxService interface and its callers
are unchanged; loudness matches the old mpv path (v^6). WAV only, so
no codec libraries are bundled and no build packages are needed.

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

apps/desktop/lib/src/platform/audio/sound_engine.dart

@@ -0,0 +1,133 @@
+import 'dart:async';
+import 'dart:typed_data';
+
+import 'package:flutter_soloud/flutter_soloud.dart';
+import 'package:sync_relay_core/core.dart';
+
+final _log = createLogger('sound');
+
+/// What [SoundEngine] needs from an audio library: [SoLoudBackend] in the
+/// app, a fake in tests. A sound handle is whatever [load] returns.
+abstract class AudioBackend {
+  Future<void> init();
+  /// Decodes [wav] (a whole WAV file) into memory under [name].
+  Future<Object> load(String name, Uint8List wav);
+  /// Starts one voice of [sound] at [volume] (linear gain, 0..1); voices
+  /// overlap.
+  void play(Object sound, double volume);
+  Future<void> unload(Object sound);
+  Future<void> deinit();
+}
+
+/// flutter_soloud (SoLoud mixing into miniaudio's output device).
+class SoLoudBackend implements AudioBackend {
+  SoLoud get _s => SoLoud.instance;
+
+  @override
+  Future<void> init() => _s.init();
+
+  @override
+  Future<Object> load(String name, Uint8List wav) => _s.loadMem(name, wav);
+
+  @override
+  void play(Object sound, double volume) => _s.play(sound as AudioSource, volume: volume);
+
+  @override
+  Future<void> unload(Object sound) => _s.disposeSource(sound as AudioSource);
+
+  @override
+  Future<void> deinit() => _s.deinitAsync();
+}
+
+/// The app's short in-memory sounds. The audio device opens on [warmUp] or
+/// the first [play]; if it can't (no device, a broken driver) the engine
+/// logs once and every play is silently dropped. Each sound is loaded once
+/// per slot from the WAV bytes its renderer gives; a new key in a slot
+/// replaces (and frees) what the slot held.
+class SoundEngine {
+  final AudioBackend _backend;
+  SoundEngine({AudioBackend? backend}) : _backend = backend ?? SoLoudBackend();
+
+  Future<bool>? _ready;
+  var _disposed = false;
+  /// Per slot: the loaded key and its sound (null: failed to load).
+  final Map<String, (String, Future<Object?>)> _slots = {};
+  /// Load names are unique: SoLoud hands back the old sound for a name it
+  /// already holds, which would defeat a re-render.
+  var _loads = 0;
+
+  /// Opens the audio device now, so the first sound has no wait.
+  Future<bool> warmUp() => _ready ??= _init();
+
+  Future<bool> _init() async {
+    if (_disposed) return false;
+    try {
+      await _backend.init();
+      return true;
+    } catch (e) {
+      _log.warning('no sounds: audio init failed: $e');
+      return false;
+    }
+  }
+
+  /// Plays the sound of [slot] at [volume] (linear gain). [key] names what
+  /// [wav] renders: the same key reuses the loaded sound; another key, or
+  /// [fresh], renders and loads it again.
+  Future<void> play(String slot, String key, Future<Uint8List> Function() wav,
+      {required double volume, bool fresh = false}) async {
+    if (_disposed || volume <= 0) return;
+    if (!await warmUp() || _disposed) return;
+    var held = _slots[slot];
+    if (held == null || held.$1 != key || fresh) {
+      if (held != null) unawaited(_unload(held.$2));
+      held = (key, _load(key, wav));
+      _slots[slot] = held;
+    }
+    final sound = await held.$2;
+    if (sound == null) {
+      // Retry on the next play.
+      if (identical(_slots[slot], held)) _slots.remove(slot);
+      return;
+    }
+    if (_disposed || !identical(_slots[slot], held)) return;
+    try {
+      _backend.play(sound, volume.clamp(0.0, 1.0));
+    } catch (e) {
+      _log.warning('sound $key: $e');
+    }
+  }
+
+  Future<Object?> _load(String key, Future<Uint8List> Function() wav) async {
+    try {
+      return await _backend.load('$key#${_loads++}', await wav());
+    } catch (e) {
+      _log.warning('sound $key: $e');
+      return null;
+    }
+  }
+
+  Future<void> _unload(Future<Object?> sound) async {
+    try {
+      final s = await sound;
+      if (s != null && !_disposed) await _backend.unload(s);
+    } catch (e) {
+      _log.fine('unload: $e');
+    }
+  }
+
+  /// Frees every sound and closes the audio device. Plays after it are
+  /// dropped.
+  Future<void> dispose() async {
+    if (_disposed) return;
+    _disposed = true;
+    _slots.clear();
+    final ready = _ready;
+    if (ready == null || !await ready) return;
+    try {
+      // Deinit frees the loaded sounds with the engine.
+      await _backend.deinit();
+    } catch (e) {
+      _log.warning('audio deinit: $e');
+    }
+  }
+}