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/default.nix

@@ -2,7 +2,7 @@
 let
   inherit (lib.my) net mkVLAN;
   inherit (lib.my.c) pubDomain;
-  inherit (lib.my.c.home) domain vlans prefixes vips hiMTU;
+  inherit (lib.my.c.home) domain vlans prefixes vips vlanDns hiMTU;
 in
 {
   imports = [ ./vms ];
@@ -203,7 +203,10 @@ in
                   MACAddress=52:54:00:90:34:95
                 '';
               };
-              "60-lan-hi" = networkdAssignment "lan-hi" assignments.hi;
+              "60-lan-hi" = mkMerge [
+                (networkdAssignment "lan-hi" assignments.hi)
+                { networkConfig = vlanDns "hi"; }
+              ];
 
               "50-lan-core-phy" = {
                 matchConfig.Name = "lan-core-phy";