Marker waits run after _run_attempt has cleared _running, so the run_loop heartbeat thread no longer refreshes the waiting parent. sweep_stale_tasks (default 300s) then kills a parent waiting up to the 3600s marker timeout, as seen in the marker live test (PM2 swept mid wait-threads). Poll and wait loops now heartbeat the parent each iteration (interval clamped to <=60s, well under the 300s threshold), best-effort, without touching _running, so no new capacity leak and cancel still works (empty _running entry has no proc/container to kill). Tests: 2 new sweep-survival tests (stale heartbeat at -299s, 2s wait, sweep must spare the parent); both fail 2/2 on pre-fix code and pass with the fix.
