Builds on jbbottoms's #65178 takeover fix (cherry-picked as the previous
commit). Windows --replace already tree-kills via taskkill /T, but the
POSIX paths signalled only the recorded gateway PID — adapter
subprocesses that outlived their parent kept holding scoped token locks
and blocked the replacement gateway.
- gateway/status.py: _snapshot_gateway_children() captures the old
gateway's descendants (psutil, recursive) while it is still alive;
reap_gateway_children() SIGTERMs verified orphans after the main PID
is confirmed dead, waits bounded, SIGKILLs survivors. Identity-aware
(psutil is_running is PID+create-time), skips zombies and children
whose ppid still equals the old gateway (parent actually alive), and
never raises — best-effort with debug/info logging only.
- take_over_scoped_lock_holder() snapshots before terminating and reaps
only on a confirmed successful handoff.
- gateway/run.py: start_gateway --replace snapshots before SIGTERM and
reaps after the old PID is confirmed gone, mirroring taskkill /T.
- tests/gateway/test_replace_child_reap.py: reap/skip/never-raise unit
coverage plus end-to-end --replace ordering (snapshot → terminate →
reap) and the no---replace path never touching the old process.
