On Linux, systemd spawns core services (cron, nginx, sshd) with
deterministic PIDs and jiffy start_times across reboots. A service can
land on the exact same PID and start_time as a previous gateway, causing
acquire_scoped_lock to mistake it for a live gateway and block startup.
The existing stale-detection paths only covered:
- start_times both non-None and different (clear mismatch)
- start_times both None (macOS/Windows fallback to cmdline check)
The boot-time collision falls through both: times are non-None and
equal, so neither branch fired.
Add a third check: when both start_times are known and match but the
live process fails _looks_like_gateway_process, read its cmdline. If
the cmdline is readable (non-None), we have positive evidence of an
impostor and mark the lock stale. Requiring a readable cmdline keeps the
check conservative — if cmdline is unreadable we do not evict.
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