Three holes in the systemd integration:
1. _spawn_agent_via_systemd only restarted decnet-agent.service, leaving
decnet-forwarder.service running the pre-update code (same /opt/decnet
tree, stale import cache).
2. run_update_self used os.execv regardless of environment — the re-execed
process kept the updater's existing cgroup/capability inheritance but
systemd would notice MainPID change and mark the unit degraded.
3. No path to surface a failed forwarder restart (legacy enrollments have
no forwarder unit).
Now: agent restart first, forwarder restart as best-effort (logged but
non-fatal so legacy workers still update), MainPID still read from the
agent unit. For update-self under systemd, spawn a detached sleep+
systemctl restart so the HTTP response flushes before the unit cycles.