nixos/portcullis: Bring up 10G on the home hi VLAN

portcullis is wired over 10G to fergal, which uplinks to jim's spare
SFP+ port. That uplink is untagged VLAN 1, so hi is carried tagged on a
lan-hi VLAN interface: a static assignment at 192.168.68.41 / ::6:1,
resolving through the router VIPs like any other hi client. Its gateway
route outranks the DHCP default, making 10G the preferred path while the
2.5G bootstrap stays as a fallback. Deploy now targets that address.

The hi MTU goes on the .network rather than the .link, since a .link is
only applied at udev device-add -- with it there, et10g-0 stays at 1500
across a switch and lan-hi cannot take 9000.

jim's sfp-spare was tagged into hi and lo out of band to match.

fergal turns out to belong with portcullis rather than to the home
fabric -- it goes to Nikhef when the box does -- so its documentation
moves to the colony site, leaving home/switches.md a short section on
what it borrows from that fabric.

Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>

docs/sites/colony/README.md

@@ -33,4 +33,7 @@ The applications running on `shill` are listed on its own page — see
 pages document only what this repository controls.
 
 `portcullis` is new hardware headed for Nikhef that will take over most of `estuary`'s edge routing.
-It is not deployed yet and the resulting topology is still being worked out.
+It is not deployed yet and the resulting topology is still being worked out. It travels with
+[`fergal`](fergal.md), an OpenWrt SFP+ switch whose firmware this flake builds; both are staged at
+home for now, borrowing the home fabric through
+[jim](../home/switches.md#fergal-portculliss-switch).