fix(player): 修复切换频道时复用已释放的视频控制器

lib/core/data/services/playback_session.dart

@@ -49,6 +49,7 @@ class PlaybackSession extends ChangeNotifier {
     this.prepare,
     this.activate,
     PlaybackCoordinator? coordinator,
+    this.waitForFrame,
   }) : _coordinator = coordinator ?? PlaybackCoordinator();
 
   final List<Episode> episodes;
@@ -60,6 +61,7 @@ class PlaybackSession extends ChangeNotifier {
 
   /// Runs only when a slot becomes current (e.g. progress listeners).
   final PlayerPreparation? activate;
+  final Future<void> Function()? waitForFrame;
   final PlaybackCoordinator _coordinator;
   final Set<_PlayerSlot> _live = {};
   _PlayerSlot? _active;
@@ -80,8 +82,16 @@ class PlaybackSession extends ChangeNotifier {
 
   int get currentIndex => _currentIndex;
   Episode get currentEpisode => episodes[_currentIndex];
-  PlayerPort? get active =>
-      _active?.index == _currentIndex ? _active?.player : null;
+  PlayerPort? get active {
+    final slot = _active;
+    if (_closed ||
+        slot == null ||
+        slot.retiring ||
+        slot.index != _currentIndex) {
+      return null;
+    }
+    return slot.player;
+  }
   bool get loading => _loading;
   bool get userWantsPlay => _userWantsPlay;
   bool get canAutoAdvance => _mayPlay && !_loading && active != null;
@@ -99,8 +109,15 @@ class PlaybackSession extends ChangeNotifier {
   /// Exposes only prepared slots so the adjacent page can mount its video texture
   /// during a swipe without starting audio or registering history listeners.
   PlayerPort? preparedPlayerAt(int index) {
+    if (_closed) return null;
     for (final slot in [_active, _next]) {
-      if (slot?.index == index && slot!.prepared) return slot.player;
+      if (slot == null ||
+          slot.retiring ||
+          slot.index != index ||
+          !slot.prepared) {
+        continue;
+      }
+      return slot.player;
     }
     return null;
   }
@@ -162,6 +179,11 @@ class PlaybackSession extends ChangeNotifier {
   Future<void> _retire(_PlayerSlot? slot) async {
     if (slot == null || !_live.contains(slot)) return;
     slot.retiring = true;
+    final cleared = _active == slot || _next == slot;
+    if (_active == slot) _active = null;
+    if (_next == slot) _next = null;
+    if (cleared) _notify();
+    await waitForFrame?.call();
     await slot.ready;
     try {
       await slot.player.pause();