playbook: document USB-C 5 Gbps NIC bridge override on netplan task

The "Configure static IP for PXE interface" task writes a flat
single-NIC netplan config. Live PXE server (10.9.100.1) overrides
this with a bridge config bonding the USB-C 5 Gbps NIC and the
onboard NIC into br-pxe, because the onboard NIC alone cannot
sustain the imaging throughput required by the floor.

Add a comment block above the task warning that re-running it on
a box already using the bridge config will replace the bridge with
a flat single-iface config and likely break the PXE LAN. The full
bridge YAML is included for reference. Recovery is via
/etc/netplan/50-cloud-init.yaml.pre-gold-swap (preserved by netplan
backup).

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

playbook/pxe_server_setup.yml

@@ -1047,6 +1047,29 @@
         name: apache2
         state: reloaded
 
+    # Single-NIC fresh-deploy default. Boxes that need higher throughput
+    # (e.g. WJF prod uses a USB-C 5 Gbps NIC) override this with a bridge
+    # config bonding the USB NIC + onboard NIC into br-pxe. Live override
+    # currently deployed on 10.9.100.1 (do NOT re-run this task there
+    # without first reviewing /etc/netplan/50-cloud-init.yaml.pre-gold-swap):
+    #
+    #   network:
+    #     version: 2
+    #     renderer: networkd
+    #     ethernets:
+    #       enp128s31f6: { dhcp4: no }
+    #       enx34c8d6b11010: { dhcp4: no }
+    #     bridges:
+    #       br-pxe:
+    #         interfaces: [enp128s31f6, enx34c8d6b11010]
+    #         addresses: [10.9.100.1/24]
+    #         parameters:
+    #           stp: false
+    #
+    # Re-running THIS task on a box already using the bridge config will
+    # replace the bridge with a single-iface flat config and likely break
+    # the PXE LAN. Skip this task or restore the .pre-gold-swap backup
+    # before applying.
     - name: "Configure static IP for PXE interface"
       copy:
         dest: /etc/netplan/50-cloud-init.yaml