channelsFromCheckpoint

Hydrate channels from a checkpoint, reconstructing any DeltaChannel whose value is absent from channel_values by replaying ancestor writes.

For most channels (and for delta channels with a DeltaSnapshot or a migrated plain value in channel_values), emptyChannels is sufficient and no saver access is required. When a delta channel is absent from channel_values, an ancestor walk via saver.getDeltaChannelHistory finds the nearest seed and accumulates the writes between it and the target. All delta channels needing replay are batched into a single saver call.