nixos/home: Recurse over IPv4 only on the home routers

The routers' IPv6 default route runs over the as211024 mesh as a
proto-static route that blackholes rather than withdrawing when the mesh
flaps (e.g. during ipsec churn on a deploy). With "::" in the recursor's
outgoing.source_address, pdns-recursor kept selecting IPv6 to reach
authoritative servers, stalled on timeouts, and took recursion down with
it whenever the mesh bounced. Drop "::" so upstream recursion goes out
the WAN over IPv4 and stays up; AAAA records are still served fine.

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

nixos/boxes/home/routing-common/dns.nix

@@ -64,7 +64,13 @@ in
           };
 
           outgoing = {
-            source_address = [ "0.0.0.0" "::" ];
+            # Query authoritative servers over IPv4 only. Our IPv6 default route runs over the
+            # as211024 mesh (`ip -6 route show default`), a proto-static route that isn't
+            # withdrawn when the mesh flaps (e.g. during ipsec churn) — it just blackholes. With
+            # "::" here the recursor keeps picking IPv6 to reach NS, stalls on timeouts, and
+            # takes recursion down with it. IPv4 upstream goes out the WAN directly and stays up;
+            # we still serve AAAA records regardless of transport.
+            source_address = [ "0.0.0.0" ];
           };
 
           recursor = {