Function dataset_unique_id

app/src/DataModel/Frame.h:1009–1014

* @brief Single source of truth for the (source, group, dataset) -> uniqueId mapping. */

 * @brief Single source of truth for the (source, group, dataset) -> uniqueId mapping.
 */
[[nodiscard]] inline constexpr int dataset_unique_id(int sourceId,
                                                     int groupId,
                                                     int datasetId) noexcept
{
  return sourceId * kSourceIdStride + groupId * kGroupIdStride + datasetId;
}

/**
 * @brief Convenience overload taking a Group + Dataset directly.