fix(player): 重构单层过渡并阻止锁定集串播

lib/core/data/services/playback_session_controls.dart

@@ -10,6 +10,26 @@ extension PlaybackSessionControls on PlaybackSession {
     _warmNeighbors(this, preferredNext: forward);
   }
 
+  /// 选中锁定集时立即停播并释放所有原生播放器，不创建新片源。
+  Future<void> selectLocked(
+    int index, {
+    int? preloadIndex,
+    int? previousIndex,
+  }) {
+    if (_closed || index < 0 || index >= episodes.length) {
+      return Future<void>.value();
+    }
+    _generation++;
+    _selected = false;
+    _currentIndex = index;
+    _rememberNeighbors(preloadIndex, previousIndex);
+    _loading = false;
+    _error = null;
+    _userWantsPlay = false;
+    _notify();
+    return _releasePlayers();
+  }
+
   /// 清晰度切换时盖住黑帧的旧播放器。只在仍由会话持有时交给界面。
   PlayerPort? handoffAt(int index) {
     final slot = _handoff;