Demux stage-2 rows per channel; produce per-channel histories.
Stage-2 rows are
(checkpoint_id, channel, task_id, idx, type, value, task_path).
Final write order is oldest→newest globally and writes_sort_key
within a checkpoint, matching the contract on
DeltaChannelHistory.writes.
seed is omitted when the walk reached a true root with no snapshot
found (channel never entered seeded); consumers treat absence as
"start empty".
