/**
 * Returns array with duplicates removed, keeping the LAST occurrence.
 * Useful for slice merging where extensions should override base slices.
 *
 * @example
 * ```ts
 * const slices = [{ id: 'a', v: 1 }, { id: 'b', v: 2 }, { id: 'a', v: 3 }];
 * uniqBy(slices, s => s.id);
 * // => [{ id: 'b', v: 2 }, { id: 'a', v: 3 }]
 * ```
 */
export function uniqBy<T, K>(arr: T[], mapper: (item: T) => K): T[] {
  const seen = new Map<K, number>();
  arr.forEach((item, i) => seen.set(mapper(item), i));
  return arr.filter((_, i) => [...seen.values()].includes(i));
}