perf(collections): reuse membership indexes for metadata changes

lib/models/library_collections.dart

@@ -171,6 +171,28 @@ class UserPlaylistCollection {
     return _trackKeys.contains(trackKey);
   }
 
+  bool sharesMembershipIndexWith(UserPlaylistCollection other) =>
+      identical(_trackKeys, other._trackKeys);
+
+  /// Publishes tracks and their index built by the hydration worker. When it
+  /// verified unchanged membership, keep the original index so the enclosing
+  /// collections state need not rescan every other playlist's tracks.
+  UserPlaylistCollection withHydratedTracks({
+    required List<CollectionTrackEntry> tracks,
+    required Set<String> trackKeys,
+    required bool membershipUnchanged,
+  }) => UserPlaylistCollection(
+    id: id,
+    name: name,
+    coverImagePath: coverImagePath,
+    createdAt: createdAt,
+    updatedAt: updatedAt,
+    tracks: tracks,
+    previewCover: previewCover,
+    tracksLoaded: true,
+    trackKeys: membershipUnchanged ? _trackKeys : trackKeys,
+  );
+
   Set<String> get trackKeys => UnmodifiableSetView(_trackKeys);
   int get trackCount => _trackKeys.length;
 
@@ -365,10 +387,30 @@ class LibraryCollectionsState {
       lovedKeys: keepLovedIndex ? _lovedKeys : null,
       favoriteArtistKeys: keepFavoriteArtistIndex ? _favoriteArtistKeys : null,
       playlistsById: keepPlaylistIndex ? _playlistsById : null,
-      allPlaylistTrackKeys: keepPlaylistIndex ? _allPlaylistTrackKeys : null,
+      allPlaylistTrackKeys:
+          keepPlaylistIndex || _hasSamePlaylistMembership(nextPlaylists)
+          ? _allPlaylistTrackKeys
+          : null,
     );
   }
 
+  bool _hasSamePlaylistMembership(List<UserPlaylistCollection> nextPlaylists) {
+    if (nextPlaylists.length != playlists.length ||
+        _playlistsById.length != playlists.length) {
+      return false;
+    }
+    final seen = <String>{};
+    for (final playlist in nextPlaylists) {
+      final previous = _playlistsById[playlist.id];
+      if (!seen.add(playlist.id) ||
+          previous == null ||
+          !identical(previous._trackKeys, playlist._trackKeys)) {
+        return false;
+      }
+    }
+    return true;
+  }
+
   Map<String, dynamic> toJson() => {
     'wishlist': wishlist.map((e) => e.toJson()).toList(),
     'loved': loved.map((e) => e.toJson()).toList(),

test/library_collections_models_test.dart

@@ -106,4 +106,37 @@ void main() {
     expect(loaded.tracksLoaded, isTrue);
     expect(loaded.trackCount, 0);
   });
+
+  test(
+    'membership remains correct after metadata copies and playlist changes',
+    () {
+      UserPlaylistCollection playlist(String id, String key) =>
+          UserPlaylistCollection(
+            id: id,
+            name: id,
+            createdAt: DateTime.utc(2026),
+            updatedAt: DateTime.utc(2026),
+            tracks: const [],
+            tracksLoaded: false,
+            trackKeys: {key},
+          );
+      final first = playlist('first', 'example:first');
+      final second = playlist('second', 'example:second');
+      final state = LibraryCollectionsState(playlists: [first, second]);
+      final renamed = state.copyWith(
+        playlists: [
+          second.copyWith(name: 'Renamed'),
+          first,
+        ],
+      );
+      expect(renamed.isTrackInAnyPlaylist('example:first'), isTrue);
+      expect(renamed.isTrackInAnyPlaylist('example:second'), isTrue);
+      final replaced = renamed.copyWith(playlists: [first, first]);
+      expect(replaced.isTrackInAnyPlaylist('example:first'), isTrue);
+      expect(replaced.isTrackInAnyPlaylist('example:second'), isFalse);
+      final removed = renamed.copyWith(playlists: [second]);
+      expect(removed.isTrackInAnyPlaylist('example:first'), isFalse);
+      expect(removed.isTrackInAnyPlaylist('example:second'), isTrue);
+    },
+  );
 }