NICs: pick the imaging port at install, GE can go into any other port

Sites no longer need either MAC in site-vars.

pxe_nic_mac empty: before anything changes, the playbook runs
pxe-pick-nic --auto. It takes the one wired port with a link, accepts it
only if pxe-nic-check calls it isolated (no LLDP/CDP/802.1X, no other
DHCP server, at most 10 hosts heard in 65 s), and pins it. The choice is
saved in /etc/pxe-server/nic-pick.conf and never re-guessed at boot.
When it cannot tell (no link, two links, not isolated) nothing changes,
and the console banner says why and to run sudo pxe-pick-nic: pull the
imaging cable and plug it back in, and the port that blinked is the
imaging one. Both helpers are installed before the pick, so that works
after a refused first boot. The site's corp domains and allowed
switches are passed to the check.

corp_nic_mac empty: every other wired port (en*, eth*) is a catch-all
DHCP client, so GE can be plugged in at install or later.
- dnsmasq stays pinned with interface=pxe0. A new check refuses any
  dhcp-range outside the imaging subnet, and proxy ranges.
- The catch-all ports ignore offers from 172.16.9.1.
- UFW allows SSH and 9009 by source on any interface, and keeps those
  rules when the old any-interface rules are removed.
- pxe_nic_name must not look like en*/eth* in this mode.

pxe-netplan-apply.sh takes 3 or 5 args. Without a pinned GE NIC it:
- renames stray names (the old pxe0 after a re-pick, a leftover corp0)
  to enx<mac>, so the catch-all takes them, and restores them on revert;
- checks that every NIC that had a DHCP address before has one after,
  tracked by MAC.

With both MACs set (the WJ server), netplan, the dnsmasq pin and UFW
render byte-for-byte as before.

Tested in the pxetest VMs:
- Fresh install with no MACs and the GE link down: the imaging port was
  picked and pinned as pxe0 with 172.16.9.1, and dnsmasq bound DHCP to
  pxe0 only. GE plugged in afterwards got a lease, SSH worked, and the
  guard reported pxe0 isolated.
- --auto refuses with two links.
- The interactive pick works by pulling and replugging the link.
- A 3-arg re-apply is UNCHANGED.

Unit tests (fake sysfs) cover the re-pick and pinned-to-catch-all renames.

playbook/templates/netplan-pxe.yaml.j2

@@ -3,6 +3,7 @@
 # replaced on the next playbook run; the previous file is kept under
 # /var/backups/netplan-<timestamp>/.
 #
+{% if corp_nic_mac %}
 # Two NICs, each matched by MAC and given a stable name:
 #   pxe-lan  imaging LAN, static, no gateway, no DNS. The ONLY NIC that
 #            serves DHCP/TFTP/HTTP boot/SMB. ignore-carrier: the address
@@ -11,6 +12,24 @@
 #            Apache without {{ pxe_server_ip }} (networkd
 #            ConfigureWithoutCarrier= + IgnoreCarrierLoss=).
 #   ge-corp  GE corporate network, DHCP client (SSH + webapp 9009, updates).
+{% else %}
+# The imaging NIC pinned by MAC, every other wired port a DHCP client:
+#   pxe-lan  imaging LAN, matched by MAC and given a stable name. Static,
+#            no gateway, no DNS. The ONLY NIC that serves DHCP/TFTP/HTTP
+#            boot/SMB. ignore-carrier: the address is put on (and kept)
+#            without link, so a switch that is off or a cable pulled at
+#            boot does not leave dnsmasq, Samba and Apache without
+#            {{ pxe_server_ip }} (networkd ConfigureWithoutCarrier= +
+#            IgnoreCarrierLoss=).
+#   zz-en, zz-eth  every other wired port: DHCP client, so the GE cable
+#            can go into any of them, at install or later. No set-name.
+#            networkd applies the first matching .network file in name
+#            order: 10-netplan-pxe-lan sorts before 10-netplan-zz-*, and
+#            {{ pxe_nic_name }} does not look like en*/eth* (the playbook
+#            checks), so the imaging NIC never falls into these. dnsmasq
+#            never serves them (interface= pins it to {{ pxe_nic_name }}),
+#            and a drop-in makes them ignore offers from {{ pxe_server_ip }}.
+{% endif %}
 network:
   version: 2
   renderer: networkd
@@ -25,6 +44,7 @@ network:
       optional: true
       addresses:
         - {{ pxe_server_ip }}/{{ pxe_prefix }}
+{% if corp_nic_mac %}
     ge-corp:
       match:
         macaddress: "{{ corp_nic_mac | lower }}"
@@ -32,3 +52,15 @@ network:
       dhcp4: true
       dhcp4-overrides:
         route-metric: {{ corp_route_metric }}
+{% else %}
+{% for id, glob in [('zz-en', 'en*'), ('zz-eth', 'eth*')] %}
+    {{ id }}:
+      match:
+        name: "{{ glob }}"
+      dhcp4: true
+      dhcp6: false
+      optional: true
+      dhcp4-overrides:
+        route-metric: {{ corp_route_metric }}
+{% endfor %}
+{% endif %}