feat(library): add lyrics filtering and metadata actions

lib/utils/ordered_range_selection.dart

@@ -0,0 +1,32 @@
+/// Adds every visible item between [anchor] and [target] to [selected].
+///
+/// Dart's default [Set] preserves insertion order, so walking backwards also
+/// preserves the direction chosen by the user when results are later copied,
+/// shared, or batch processed.
+T addOrderedSelectionRange<T>({
+  required Set<T> selected,
+  required List<T> visibleItems,
+  required T target,
+  T? anchor,
+}) {
+  final anchorIndex = anchor == null ? -1 : visibleItems.indexOf(anchor);
+  final targetIndex = visibleItems.indexOf(target);
+  if (anchorIndex < 0 || targetIndex < 0) {
+    selected.add(target);
+    return target;
+  }
+
+  final step = targetIndex >= anchorIndex ? 1 : -1;
+  for (var index = anchorIndex; ; index += step) {
+    selected.add(visibleItems[index]);
+    if (index == targetIndex) break;
+  }
+  return target;
+}
+
+/// Toggles one item and returns the next range-selection anchor.
+T? toggleOrderedSelection<T>({required Set<T> selected, required T target}) {
+  if (selected.add(target)) return target;
+  selected.remove(target);
+  return selected.isEmpty ? null : selected.last;
+}