DeltaSnapshot

Snapshot blob for a DeltaChannel with finite snapshot frequency.

Stored directly in a checkpoint's channel_values in place of the full accumulated value. The ancestor walk in BaseCheckpointSaver.getDeltaChannelHistory terminates when it encounters a populated channel_values entry for a channel; a DeltaSnapshot value is the materialized state at that ancestor, so the channel reconstructs directly from .value without replaying earlier writes.