Function merge_snapshot_into_state 

fn merge_snapshot_into_state(
    existing: Option<&HostRolloutState>,
    snapshot: HostRolloutSnapshot,
) -> HostRolloutState

Pure merge of a wire snapshot with the existing in-memory entry. Canonical fields (state, target_closure, dispatch/activation timestamps, last_event_seq) come from the snapshot. Agent-local fields not carried in the wire shape are preserved from existing when present, defaulted when not.