The non-blocking spawn (start_intake_session / start_secretary_session)
schedules _spawn_intake_container_guarded / _spawn_secretary_container_guarded
via _schedule_bg. Those run docker run and only register in _instances at the
END. If shutdown arrived between docker run and the registration line, the
container was started but the orchestrator had no handle — stop() iterates
only _instances, so the container was orphaned (leaked, manual docker rm).
Worse, the F070 drain could let the spawn coroutine complete the
registration AFTER stop() already iterated _instances, landing a live
container into a shutting-down registry nothing tears down.
Add a post-docker-run shutdown guard in _spawn_intake_container and
_spawn_secretary_container: re-check self._running after _run_container_cmd
returns; if the orchestrator began shutting down, remove the just-started
container (by its deterministic name) and raise _SpawnAbortedDuringShutdown
WITHOUT registering. The guarded wrappers catch that BEFORE except Exception
and close the live relay silently (shutdown is not a user-facing failure,
no error pushed to the SSE stream). The F070 stop() drain awaits the bg
spawn coroutine, so the abort surfaces cleanly.
TOCTOU-safe: between the _running check and the _instances assignment there
is no await (config + instance construction are sync), so once the check
passes, registration completes before the event loop can interleave stop().
The normal running path is unchanged (sanity tests pin it).
