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/sfh/default.nix

@@ -1,7 +1,7 @@
 { lib, ... }:
 let
   inherit (lib.my) net;
-  inherit (lib.my.c.home) domain prefixes vips hiMTU roceBootModules;
+  inherit (lib.my.c.home) domain prefixes vips vlanDns hiMTU roceBootModules;
 in
 {
   imports = [ ./containers ];
@@ -134,8 +134,10 @@ in
           networks = {
             "30-lan-hi" = mkMerge [
               (networkdAssignment "lan-hi" assignments.hi)
-              # So we don't drop the IP we use to connect to NVMe-oF!
-              { networkConfig.KeepConfiguration = "static"; }
+              {
+                # So we don't drop the IP we use to connect to NVMe-oF!
+                networkConfig = vlanDns "hi" // { KeepConfiguration = "static"; };
+              }
             ];
             "30-lan-hi-ctrs" = {
               matchConfig.Name = "lan-hi-ctrs";