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.

Pre-delta plain-value blobs subsume their ancestor's pending writes (the value already includes them); _DeltaSnapshot blobs do not (snapshot is the value AT that ancestor, prior to its own pending writes that produce the child).