nixos/home: Anchor static hi clients' DNS on VIPs

Statically-addressed home servers on hi run no DHCP, so they learned a
resolver only from the v6 RA RDNSS and lost DNS whenever v6 (and thus
the RA) was absent. Factor the fix castle/palace applied inline into a
shared lib.my.c.home.vlanDns helper that points resolved at the VLAN's
VRRP VIPs (always-present static v4, plus v6 when up) and sets the
advertised search domains, then apply it to every statically-addressed
hi client: castle, palace, cellar, sfh and the sfh hass/unifi
containers. Document it under the router client DNS section.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

nixos/boxes/home/palace/vms/cellar/default.nix

@@ -2,7 +2,7 @@
 let
   inherit (lib.my) net;
   inherit (lib.my.c) pubDomain;
-  inherit (lib.my.c.home) domain prefixes vips hiMTU;
+  inherit (lib.my.c.home) domain prefixes vips vlanDns hiMTU;
 in
 {
   nixos.systems.cellar = {
@@ -79,7 +79,10 @@ in
               };
 
               networks = {
-                "80-lan-hi" = networkdAssignment "lan-hi" assignments.hi;
+                "80-lan-hi" = mkMerge [
+                  (networkdAssignment "lan-hi" assignments.hi)
+                  { networkConfig = vlanDns "hi"; }
+                ];
               };
             };