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>

lib/constants.nix

@@ -385,6 +385,16 @@ rec {
       };
     };
 
+    # networkConfig fragment anchoring a VLAN client's DNS on the router pair's
+    # VRRP VIPs rather than the RA RDNSS. v6 addresses here are RA/token-derived,
+    # so when RA is absent (e.g. v6 disabled) there is no v6 and no RDNSS at all;
+    # the always-present static v4 VIP keeps name resolution working, with the v6
+    # VIP as a bonus when v6 is up. Merge into the VLAN network's networkConfig.
+    vlanDns = vlan: {
+      DNS = [ vips.${vlan}.v4 vips.${vlan}.v6 ];
+      Domains = searchDomains;
+    };
+
     roceBootModules = [ "ib_core" "ib_uverbs" "mlx5_core" "mlx5_ib" ];
   };