perf(library): reuse collection lookups across selection rebuilds

test/library_tracks_folder_data_test.dart

@@ -0,0 +1,65 @@
+import 'dart:collection';
+import 'package:spotiflac_android/models/track.dart';
+import 'package:spotiflac_android/providers/library_collections_provider.dart';
+import 'package:spotiflac_android/screens/library_tracks_folder_data.dart';
+import 'package:spotiflac_android/services/history_database.dart';
+import 'package:flutter_test/flutter_test.dart';
+
+class _ReadCountingList<T> extends ListBase<T> {
+  _ReadCountingList(this.values);
+  final List<T> values;
+  int reads = 0;
+  @override
+  int get length => values.length;
+  @override
+  set length(int value) => throw UnsupportedError('immutable');
+  @override
+  T operator [](int index) {
+    reads++;
+    return values[index];
+  }
+
+  @override
+  void operator []=(int index, T value) => throw UnsupportedError('immutable');
+}
+
+void main() {
+  TestWidgetsFlutterBinding.ensureInitialized();
+  test('unchanged collection snapshot does no additional full-list reads', () {
+    final entries = _ReadCountingList(
+      List.generate(
+        4000,
+        (index) => CollectionTrackEntry(
+          key: '$index',
+          addedAt: DateTime.utc(2026),
+          track: Track(
+            id: '$index',
+            name: 'Song $index',
+            artistName: 'Artist',
+            albumName: 'Album',
+            duration: 1,
+          ),
+        ),
+      ),
+    );
+    final data = LibraryTracksFolderData()..update(entries);
+    final request = data.historyRequest;
+    final reads = entries.reads;
+    final hash = request.hashCode;
+    for (var i = 0; i < 100; i++) {
+      data.update(entries);
+      expect(identical(data.historyRequest, request), isTrue);
+      expect(data.historyRequest.hashCode, hash);
+    }
+    expect(entries.reads, reads);
+    expect(data.keys.length, 4000);
+    expect(() => data.tracks.clear(), throwsUnsupportedError);
+    expect(() => request.tracks.clear(), throwsUnsupportedError);
+    expect(request, HistoryBatchLookupRequest(request.tracks));
+    expect(hash, HistoryBatchLookupRequest(request.tracks).hashCode);
+    data.update(entries.take(2).toList());
+    expect(data.keys, {'0', '1'});
+    expect(data.tracks.length, 2);
+    expect(identical(data.historyRequest, request), isFalse);
+  });
+}