NIC guard: dnsmasq never serves DHCP/PXE on the GE network

pxe-nic-check classifies each NIC from what it hears: LLDP/CDP/802.1X
from managed switches, foreign DHCP offers and their domain (one
DHCPDISCOVER on the imaging NIC; the GE NIC's networkd lease is read
instead), STP and ARP hosts for information. A kernel BPF filter keeps
bulk SMB traffic out of the capture loop; bridge ports are listened on
too, since LLDP and EAPOL stop at the port.

- dnsmasq drop-in runs --guard as ExecStartPre: exit 1 on a ge or
  foreign-dhcp imaging verdict, fail open (loudly) on bugs or no link
- pxe-nic-check.timer runs --watch every 5 min: stops dnsmasq on a bad
  imaging verdict, never starts it
- /run/pxe-nic-status.json (0644, atomic) for the webapp
- playbook installs it all and writes /etc/pxe-server/nic.conf from
  pxe_imaging_mac, pxe_ge_mac, pxe_corp_domain_suffixes,
  pxe_nic_allowed_switches
- unit tests plus a no-root network-namespace lab (veth pairs, a raw
  socket DHCP/LLDP peer and real dnsmasq); docs/pxe-nic-guard.md has the
  operator notes and the two floor tests

playbook/pxe-server-helpers/pxe-nic-check.service

@@ -0,0 +1,16 @@
+# Periodic imaging-NIC watch (pxe-nic-check.timer runs this every 5 min).
+# Listens 60 s on each NIC, probes the imaging NIC for foreign DHCP, writes
+# /run/pxe-nic-status.json for the webapp, and STOPS dnsmasq if the imaging
+# NIC turns out to be on the GE network or next to another DHCP server.
+# It never starts dnsmasq: after fixing the cabling, start it by hand
+# (systemctl start dnsmasq), which re-runs the guard.
+
+[Unit]
+Description=PXE NIC watch: keep dnsmasq off the GE corporate network
+After=network.target
+
+[Service]
+Type=oneshot
+ExecStart=/usr/local/sbin/pxe-nic-check --watch
+TimeoutStartSec=180
+Nice=5