/// Unions [remote] and [local] by ID, keeping whichever copy of each
/// record has the newer `updatedAt` when a record exists on both sides.
/// This lets two devices' independent new records both survive a sync;
/// it does not propagate deletions (see plan doc's Known limitations).
List<T> mergeById<T>(
  List<T> remote,
  List<T> local,
  String Function(T) idOf,
  DateTime Function(T) updatedAtOf,
) {
  final merged = <String, T>{};
  for (final item in remote) {
    merged[idOf(item)] = item;
  }
  for (final item in local) {
    final id = idOf(item);
    final existing = merged[id];
    if (existing == null || !updatedAtOf(existing).isAfter(updatedAtOf(item))) {
      merged[id] = item;
    }
  }
  return merged.values.toList();
}