After restarting a service-managed gateway (systemd/launchd), the
stale-process sweep calls find_gateway_pids() which returns ALL gateway
PIDs via ps aux — including the one just spawned by the service manager.
The sweep kills it, leaving the user with a stopped gateway and a
confusing 'Restart manually' message.
Fix: add _get_service_pids() to query systemd MainPID and launchd PID
for active gateway services, then exclude those PIDs from the sweep.
Also add exclude_pids parameter to find_gateway_pids() and
kill_gateway_processes() so callers can skip known service-managed PIDs.
Adds 9 targeted tests covering:
- _get_service_pids() for systemd, launchd, empty, and zero-PID cases
- find_gateway_pids() exclude_pids filtering
- cmd_update integration: service PID not killed after restart
- cmd_update integration: manual PID killed while service PID preserved
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