getDeltaChannelHistory

Override: walk the parent chain ONCE for all requested channels using direct storage access.

Each channel terminates independently at the nearest ancestor whose stored channel_values[ch] is populated. Other channels keep walking until they find their own terminator or hit the root.

The seed value (whether a DeltaSnapshot or a plain pre-delta migration blob) is the value AT that ancestor, prior to its own pending writes that produce the child. Those on-path writes — including the ones stored on the terminating ancestor — are always collected and replayed on top of the seed, so a thread migrated from a pre-delta channel does not drop the writes saved under the migration boundary checkpoint.