Each Steps row now carries a variable-length child_track_ids int array
containing the track IDs of all secondaries spawned during that step.
The parent_id column is removed — parentage is now expressed
unidirectionally (parent→children) via child_track_ids and the Spawning
ntuple, rather than bidirectionally.
Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>