feat(audio): add Kodi-style stereo downmix with center channel boost

lib/mpv/player/player_native.dart

@@ -288,7 +288,7 @@ class PlayerNative extends PlayerBase {
       await _applyPassthrough(false);
     }
     await setProperty('speed', rate.toString());
-    if (_passthroughRequested && !_passthroughActive && rate == 1.0) {
+    if (_passthroughRequested && !_passthroughActive && rate == 1.0 && !_downmixEnabled) {
       await _applyPassthrough(true);
     }
   }
@@ -364,6 +364,7 @@ class PlayerNative extends PlayerBase {
   bool _passthroughRequested = false;
   bool _passthroughActive = false;
   bool _normalizationRequested = false;
+  bool _downmixEnabled = false;
   double _currentRate = 1.0;
 
   @override
@@ -377,8 +378,9 @@ class PlayerNative extends PlayerBase {
   @override
   Future<void> setAudioPassthrough(bool enabled) async {
     _passthroughRequested = enabled;
-    // Deferred until the rate returns to 1.0 (see setRate).
-    if (enabled && _currentRate != 1.0) return;
+    // Deferred until the rate returns to 1.0 (see setRate) and the stereo
+    // downmix ends (see setAudioDownmix).
+    if (enabled && (_currentRate != 1.0 || _downmixEnabled)) return;
     await _applyPassthrough(enabled);
   }
 
@@ -408,6 +410,20 @@ class PlayerNative extends PlayerBase {
     await super.setAudioNormalization(enabled);
   }
 
+  @override
+  Future<void> setAudioDownmix({required bool enabled, required int centerBoostDb, required bool normalize}) async {
+    _downmixEnabled = enabled;
+    // spdif bypasses the filter chain entirely; passthrough yields while a
+    // stereo downmix is forced and returns when it is disabled.
+    if (enabled && _passthroughActive) {
+      await _applyPassthrough(false);
+    }
+    await super.setAudioDownmix(enabled: enabled, centerBoostDb: centerBoostDb, normalize: normalize);
+    if (!enabled && _passthroughRequested && !_passthroughActive && _currentRate == 1.0) {
+      await _applyPassthrough(true);
+    }
+  }
+
   @override
   Future<void> updateFrame() async {
     if (disposed || !initialized) return;