The session-hygiene compressor runs on the loop's default executor
(run_in_executor(None, ...)), which the self._executor quiesce never joins.
It was only registered with _track_deferred_agent_worker once a timeout,
turn-hold or unwind deferred it, so a gateway stop() that reached the
SessionDB close while the awaiting turn was still waiting saw zero deferred
workers and closed/checkpointed state.db under the worker's late write
(#101064 shape).
Track the future right after run_in_executor, mirroring
run_codex_hygiene_compaction. The existing close guard (#102198) now skips
close:session_db while the worker is live, and shutdown's interrupt pass
reaches the hygiene agent. _defer_agent_cleanup_until_future_done is kept
for cleanup; re-tracking the same future is idempotent (dict key).
The default executor is deliberately NOT shut down: stop() still uses
asyncio.to_thread afterwards (terminal runtime-status flush).
Regression test adapted from @pmaho's
test_default_executor_worker_is_seen_by_the_close_guard (#121360), now
driving the real _hmwa_hygiene_detached_attempt submission site.
Co-authored-by: pmaho <42786356+pmaho@users.noreply.github.com>
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