create_checkpoint

Build a new Checkpoint from the previous one and live channel state.

For each name in channels_to_snapshot, a _DeltaSnapshot(value) blob is written into channel_values[k]. Other delta channels are omitted from channel_values — the ancestor walk reconstructs their state from checkpoint_writes. Callers compute the set via delta_channels_to_snapshot(channels, counters); defaults to empty (no snapshots) when not provided.