Process one streamed stage-1 row in the merged ancestor walk.
The cursor returns (cid, type, blob) rows in walk order starting at target. The first row is target itself and is skipped (target's own writes/seed are not part of the contract).
For each subsequent row we deserialize the blob, append the cid to
every not-yet-seeded channel's chain, and check channel_values for
seeds. The deserialized checkpoint is dropped before advancing — no
cross-row cache, so peak in-flight is one deserialized checkpoint.
Returns True when the caller can stop iterating and close the cursor: every requested channel is seeded, or the chain revisited a checkpoint.
