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.

SETUP.md

@@ -144,9 +144,15 @@ This creates a bootable USB with:
 
 ### Step 5: Connect to Isolated Network
 
-Both NICs are cabled before the install: the imaging NIC (`pxe_nic_mac`)
-to the isolated switch for PXE clients, the corp NIC (`corp_nic_mac`) to
-the GE network. Nothing needs to move after the install.
+During the install plug in ONLY the imaging switch (GE unplugged). With
+`pxe_nic_mac` empty in site-vars, the first boot picks the one wired port
+with a link, checks it is isolated (no GE switch, no 802.1X, no other DHCP
+server) and pins it. Then plug the GE cable into any other port: it becomes
+a DHCP client for SSH and the webapp, and dnsmasq never serves it. If the
+first boot cannot tell which port is the imaging one, the console says so:
+log in and run `sudo pxe-pick-nic`, pull the imaging cable and plug it back
+in. Sites that prefer to pin both ports by MAC set `pxe_nic_mac` and
+`corp_nic_mac` instead; both cables can then be in from the start.
 
 ### Step 6: Import WinPE Content (if not bundled in Step 3)