perf(library): dispose unused playback row subscriptions

Auto-dispose media-item playback providers after their last listener leaves so scrolling does not retain every visited row subscription. Test disposal of 200 rows, active-row behavior and reattachment.

lib/providers/music_player_provider.dart

@@ -86,22 +86,24 @@ final playbackLoadingProvider = Provider<bool>((ref) {
 /// Transport state for one media item. Non-current Library cells only watch
 /// the current media ID; the active cell additionally follows play/loading so
 /// a transport toggle does not rebuild the entire Library grid.
-final mediaItemPlaybackUiProvider =
-    Provider.family<({bool isCurrent, bool isPlaying, bool isLoading}), String>(
-      (ref, mediaId) {
-        final currentId = ref.watch(
-          currentMediaItemProvider.select((state) => state.value?.id),
-        );
-        if (mediaId.isEmpty || currentId != mediaId) {
-          return (isCurrent: false, isPlaying: false, isLoading: false);
-        }
-        return (
-          isCurrent: true,
-          isPlaying: ref.watch(playbackPlayingProvider),
-          isLoading: ref.watch(playbackLoadingProvider),
-        );
-      },
-    );
+/// Release each cell's subscription when it scrolls out of the Library.
+final mediaItemPlaybackUiProvider = Provider.autoDispose
+    .family<({bool isCurrent, bool isPlaying, bool isLoading}), String>((
+      ref,
+      mediaId,
+    ) {
+      final currentId = ref.watch(
+        currentMediaItemProvider.select((state) => state.value?.id),
+      );
+      if (mediaId.isEmpty || currentId != mediaId) {
+        return (isCurrent: false, isPlaying: false, isLoading: false);
+      }
+      return (
+        isCurrent: true,
+        isPlaying: ref.watch(playbackPlayingProvider),
+        isLoading: ref.watch(playbackLoadingProvider),
+      );
+    });
 
 final playQueueProvider = StreamProvider<List<MediaItem>>((ref) {
   return musicPlayerQueueEvents();