nixos/home: Serve DNS on VRRP VIP and fix ipsec WAN ordering

Clients were handed both routers' real addresses as resolvers, so a downed
router meant per-query resolver timeouts. Serve pdns-recursor on the VRRP
VIPs (with non-local bind so the backup can pre-bind them) and advertise the
VIP via kea and radvd, so DNS follows the master. untrusted advertises
Cloudflare over v6 to match its v4 config.

ipsec started before the WAN's public IP was up: stream's wan carries a
static modem address that satisfies wait-online before the DHCP lease, so
libreswan loaded its mesh conns (left=<public IP>) unoriented and never
initiated. Gate stream's wan-online.target on the DHCP default route
instead, and mkForce ipsec onto wan-online.target only (dropping the
strongswan/libreswan multi-user.target pull-in) so the gate actually holds
on both boxes.

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

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

@@ -13,6 +13,13 @@ let
 in
 {
   config = {
+    # Let pdns-recursor bind the VRRP VIPs even on the backup, where the addresses
+    # aren't present locally
+    boot.kernel.sysctl = {
+      "net.ipv4.ip_nonlocal_bind" = 1;
+      "net.ipv6.ip_nonlocal_bind" = 1;
+    };
+
     my = {
       secrets.files = {
         "home/pdns/auth.conf" = {
@@ -40,6 +47,10 @@ in
               "127.0.0.1" "::1"
               assignments.hi.ipv4.address assignments.hi.ipv6.address
               assignments.lo.ipv4.address assignments.lo.ipv6.address
+              # VRRP VIPs: DNS follows the master, so clients only ever have one
+              # (always-live) resolver address and never hang on a dead router
+              vips.hi.v4 vips.hi.v6
+              vips.lo.v4 vips.lo.v6
             ];
             allow_from = [
               "127.0.0.0/8" "::1/128"