Patch form of a fingerprinted state envelope, against the last one sent to
this seat. None the first time a seat is served, or if anything is missing,
and the caller falls back to the full state, which is always correct.
When the engine host last received a player’s response, as milliseconds on
its own monotonic clock. The response path writes it and the emit path
reads it, so an outgoing envelope can say how long the engine took without
anyone comparing clocks across hosts.