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.

test/music_player_provider_test.dart

@@ -1,3 +1,5 @@
+import 'dart:async';
+
 import 'package:audio_service/audio_service.dart';
 import 'package:flutter_riverpod/flutter_riverpod.dart';
 import 'package:flutter_test/flutter_test.dart';
@@ -35,4 +37,70 @@ void main() {
       isLoading: false,
     ));
   });
+
+  test(
+    'Library releases playback state for cells that leave the viewport',
+    () async {
+      final mediaItems = StreamController<MediaItem?>();
+      addTearDown(mediaItems.close);
+      final container = ProviderContainer(
+        overrides: [
+          currentMediaItemProvider.overrideWith((ref) => mediaItems.stream),
+          playbackPlayingProvider.overrideWith((ref) => true),
+          playbackLoadingProvider.overrideWith((ref) => false),
+        ],
+      );
+      addTearDown(container.dispose);
+      final visibleProvider = mediaItemPlaybackUiProvider('visible-track');
+      final visibleSubscription = container.listen(visibleProvider, (_, _) {});
+      addTearDown(visibleSubscription.close);
+      mediaItems.add(
+        const MediaItem(id: 'visible-track', title: 'Visible track'),
+      );
+      await container.read(currentMediaItemProvider.future);
+
+      for (var index = 0; index < 200; index++) {
+        final provider = mediaItemPlaybackUiProvider('scrolled-track-$index');
+        final subscription = container.listen(provider, (_, _) {});
+        expect(container.exists(provider), isTrue);
+        subscription.close();
+      }
+      await container.pump();
+
+      for (var index = 0; index < 200; index++) {
+        expect(
+          container.exists(
+            mediaItemPlaybackUiProvider('scrolled-track-$index'),
+          ),
+          isFalse,
+        );
+      }
+      expect(container.exists(visibleProvider), isTrue);
+      expect(visibleSubscription.read(), (
+        isCurrent: true,
+        isPlaying: true,
+        isLoading: false,
+      ));
+
+      mediaItems.add(
+        const MediaItem(id: 'scrolled-track-0', title: 'Scrolled track'),
+      );
+      await container.pump();
+      final returningSubscription = container.listen(
+        mediaItemPlaybackUiProvider('scrolled-track-0'),
+        (_, _) {},
+      );
+      addTearDown(returningSubscription.close);
+      expect(returningSubscription.read(), (
+        isCurrent: true,
+        isPlaying: true,
+        isLoading: false,
+      ));
+      expect(visibleSubscription.read(), (
+        isCurrent: false,
+        isPlaying: false,
+        isLoading: false,
+      ));
+    },
+  );
 }