nixos/castle: Set lan-lo to the standard MTU

lan-lo is a VLAN on et100g, which carries hi's jumbo frames, so it
inherited 9000 rather than the 1500 the lo VLAN runs at. MSS clamping
hid this from TCP; UDP without working PMTUD was silently dropped.

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

nixos/boxes/home/castle/default.nix

@@ -198,6 +198,9 @@ in
               ];
               "45-lan-lo" = {
                 matchConfig.Name = "lan-lo";
+                # The parent carries hi's jumbo frames, but lo runs at the standard MTU;
+                # without this the VLAN would inherit the parent's larger one.
+                linkConfig.MTUBytes = "1500";
                 networkConfig = {
                   DHCP = "ipv4";
                   IPv6AcceptRA = true;