DEPLOY.md: troubleshooting for status=226/NAMESPACE failure

The 226/NAMESPACE with 'Failed to set up mount namespacing' error
is misleading — the binary is fine, but systemd can't build the
mount namespace for the service. The binary runs fine when
launched directly as administrator; the bug is in the systemd
manager's runtime state.

Document the diagnostic (run the binary manually) and the two
fixes: systemctl daemon-reexec (recreate /run/systemd) as the
first attempt, reboot as the last resort.

DEPLOY.md

@@ -255,3 +255,27 @@ docker load -i /path/to/php-8.3-apache.tar
 docker load -i /path/to/alpine-3.20.tar
 exit
 ```
+
+### Service fails with `status=226/NAMESPACE` and `Failed to set up mount namespacing: No such file or directory`
+
+Your binary is fine; systemd's service-execution environment is broken. Diagnose by running the binary manually as `administrator`:
+
+```bash
+ssh administrator@172.18.139.186
+./SDP/bin/control-plane -addr :3452 -data ./SDP/data
+# Should print "control-plane listening on :3452 (data=./SDP/data)"
+# Ctrl-C to exit
+exit
+```
+
+If that works, the binary is fine. systemd's namespace setup is failing — common cause on this Ubuntu: `/run/systemd` is missing. Force it to be recreated:
+
+```bash
+ssh administrator@172.18.139.186
+sudo systemctl daemon-reexec
+sudo systemctl restart sdp-control-plane.service
+sudo systemctl --no-pager status sdp-control-plane.service | head -10
+exit
+```
+
+If still failing, the systemd manager itself is in a bad state. Reboot the VM (last resort; will interrupt any other work on it).