Each cycle step (kickstart, update, /clear, /init) checks for `stopped` before
`await session.writeViaMux(...)`, then emits `stepSent` and calls
`setState('waiting_*')` after it.
stop() is asynchronous with respect to that await. One that lands while the
write is in flight has already passed the guard that ran, so the post-await
setState() puts a stopped controller back into a waiting state — re-arming its
step timers against a session the user asked to stop.
Re-check after the await, before emitting and setting state.
The guard reads the public `state` getter rather than `_state` on purpose:
TypeScript narrows `_state` across the await from the pre-await check and cannot
see that stop() mutated it, so `this._state === 'stopped'` is rejected as a
comparison with no overlap (TS2367) at all four sites.
Adds test/respawn-stop-race.test.ts, which drives the interleaving
deterministically by calling stop() from inside the mocked write rather than
relying on timing. All four steps go red without these guards.
