feat(library): add lyrics filtering and metadata actions

test/issue_542_utils_test.dart

@@ -0,0 +1,55 @@
+import 'dart:convert';
+
+import 'package:flutter_test/flutter_test.dart';
+import 'package:spotiflac_android/services/backup_service.dart';
+import 'package:spotiflac_android/utils/isrc_utils.dart';
+import 'package:spotiflac_android/utils/ordered_range_selection.dart';
+
+void main() {
+  group('issue 542 utility behavior', () {
+    test('formats valid ISRC for display without changing canonical value', () {
+      expect(formatIsrcForDisplay('aa9bq2200061'), 'AA-9BQ-22-00061');
+      expect(normalizeIsrc('AA-9BQ-22-00061'), 'AA9BQ2200061');
+      expect(canonicalIsrcForCopy('aa-9bq-22-00061'), 'AA9BQ2200061');
+      expect(formatIsrcForDisplay('not-an-isrc'), 'not-an-isrc');
+      expect(canonicalIsrcForCopy('not-an-isrc'), 'not-an-isrc');
+    });
+
+    test('range selection follows the user direction and insertion order', () {
+      final selected = <String>{'d'};
+
+      final anchor = addOrderedSelectionRange(
+        selected: selected,
+        visibleItems: const ['a', 'b', 'c', 'd', 'e'],
+        anchor: 'd',
+        target: 'b',
+      );
+
+      expect(anchor, 'b');
+      expect(selected.toList(), const ['d', 'c', 'b']);
+    });
+
+    test('single selection toggle keeps the latest selected anchor', () {
+      final selected = <String>{};
+      expect(toggleOrderedSelection(selected: selected, target: 'a'), 'a');
+      expect(toggleOrderedSelection(selected: selected, target: 'c'), 'c');
+      expect(toggleOrderedSelection(selected: selected, target: 'c'), 'a');
+      expect(selected.toList(), const ['a']);
+    });
+
+    test('backup encoding is indented and remains parseable', () {
+      final encoded = BackupService.encode({
+        'magic': BackupService.magic,
+        'data': {
+          'history': [
+            {'id': 'one'},
+          ],
+        },
+      });
+
+      expect(encoded, contains('\n  "magic"'));
+      expect(encoded, contains('\n    "history"'));
+      expect(jsonDecode(encoded), isA<Map<String, dynamic>>());
+    });
+  });
+}