channels_from_checkpoint

Hydrate channels from a checkpoint.

For most channels, spec.from_checkpoint(checkpoint["channel_values"][k]) is sufficient. DeltaChannel is the exception: when the channel is absent from channel_values, an ancestor walk via saver.get_delta_channel_history is required to find the nearest seed (_DeltaSnapshot blob or pre-migration plain value) and accumulate the writes between it and the target. All delta channels needing replay are batched into a single saver call.