LOAD_STATS_METHOD]: what the load cost, sent after the replay
has been written to the client and before the session/load response.
node_count / payload_bytes repeat the whole-session totals from the
intent event so this notification is self-sufficient. loaded_* are what
the agent actually parsed for this replay; for a full v1 replay they only
differ from the totals by the unreachable (dead-branch) rows the lazy
loader skips, and under a future tail replay they would be the tail.