start_drain_thread and start_session_channel_reaper checked is_alive()
and then created + started the daemon thread without holding a lock, a
check-then-act race: two concurrent callers could both see "not alive"
and each spawn a thread. The loser's thread was never stored in the
module global and ran forever, un-joinable by the matching stop_*. Both
check-then-start sequences now run under a dedicated
_THREAD_LIFECYCLE_LOCK (kept separate from the purpose-bound
SESSION_CHANNELS_LOCK / _EMIT_COALESCE_LOCK), so exactly one thread is
ever created.
Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>
