get_delta_channel_history

Override: walk the parent chain ONCE for all requested channels.

Each channel terminates independently at the nearest ancestor whose stored blob is non-empty. Other channels keep walking until they find their own terminator or hit the root.

A blob is the value AT its ancestor, prior to the writes stored under that same ancestor (those writes produce its child, which is on the path to the target). This holds for _DeltaSnapshot blobs and for pre-delta plain values alike, so the seed ancestor's own writes are always collected. Writes at ancestors older than the seed are subsumed by the seed value and are never reached — the walk terminates there.