audit finding #16. steady state was a full SectorState (21903) every
four ticks - a hard client reset each time - and the per-turn channel
the client is actually written around was never driven.
added SectorHeartbeatMessage (21902): wire is vint serverTurn, vint
checksum, with optional command and server-tick-data blocks the client's
decode treats as absent at end of stream. the session now emits one per
turn (every 10 ticks); the client reads serverTick as 10*serverTurn and
verifies its predicted checksum for that tick against ours via
LogicGameMode::endTurnReceivedFromServer. a match keeps it in sync with
no reset; a mismatch flips it out of sync, which makes it request a
sector state - which request_sector_state already serves. now that the
idle divergence is fixed the early-game turns verify cleanly.
format confirmed against SectorHeartbeatMessage::decode/encode and
getMessageType (21902) in the client. the full snapshots still go out
too; routing the bot's plays as commands inside the heartbeat and then
dropping the periodic snapshot is the next step, but this stands up the
channel and the checksum verification.
harness: the session emits heartbeats at ticks 10 and 20, and the first
decodes to (turn 1, checksum_at(10)).
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