Add the home gateway config and tunnel source addresses

- wireguard.<n>.sourceAddresses: source for router-originated traffic
  into a tunnel without an address of its own (PreferredSource on the
  routes); checked at build time.
- examples/home: antigua's routing role ported to the router module
  (PPPoE with IPv6 inside the session, LAN with ULA and reservation,
  local DNS and forward zones, the freifunk/family/mobile tunnels with
  explicit zone rules, ddclient following the PPP device).
  nixosConfigurations.home, packages.installer-home and a flake check
  that builds it.
- PLAN.md: the remaining antigua gaps and the open question about
  services that stay behind the router.

Co-Authored-By: Claude Sonnet 5.5 <noreply@anthropic.com>

modules/router/assertions.nix

@@ -380,6 +380,28 @@ let
         assertion = lib.stringLength w.interface <= 15;
         message = "router: ${where}: device name `${w.interface}` is longer than 15 characters.";
       }
+      {
+        assertion = lib.all anyIp w.sourceAddresses;
+        message = "router: ${where}.sourceAddresses must be IPv4/IPv6 addresses.";
+      }
+      {
+        assertion =
+          lib.length (lib.filter isV6 w.sourceAddresses) <= 1
+          && lib.length (lib.filter (s: !isV6 s) w.sourceAddresses) <= 1;
+        message = "router: ${where}.sourceAddresses may hold at most one address per family.";
+      }
+      {
+        assertion = lib.all (
+          s:
+          isV6 s
+          || !(validIp s)
+          || lib.elem s (
+            map (l: l.ipv4.address) lans
+            ++ map (a: builtins.head (lib.splitString "/" a)) (lib.concatMap (x: x.addresses) wgs)
+          )
+        ) w.sourceAddresses;
+        message = "router: ${where}.sourceAddresses: an IPv4 source must be the router's address on a LAN or a tunnel.";
+      }
       {
         assertion = lib.all validCidr w.addresses;
         message = "router: ${where}.addresses must be IPv4/IPv6 addresses in CIDR notation.";

modules/router/options.nix

@@ -296,6 +296,19 @@ in
                   empty means no transfer network (fine for site-to-site).
                 '';
               };
+              sourceAddresses = mkOption {
+                type = types.listOf types.str;
+                default = [ ];
+                example = [ "192.168.10.1" ];
+                description = ''
+                  Source address (at most one per family) for traffic the router
+                  itself sends into this tunnel, e.g. DNS queries to the other
+                  site. Needed when the tunnel has no address of its own, so the
+                  other side sees a source it routes back instead of whatever the
+                  kernel picks. IPv4 ones must be an address the router holds on
+                  a LAN or a tunnel.
+                '';
+              };
               zone = mkOption {
                 type = types.str;
                 default = name;

modules/router/wireguard.nix

@@ -64,7 +64,19 @@ in
           IPv6AcceptRA = false;
         };
         # Cryptokey routing: a peer's allowedIPs are the destinations sent into the tunnel.
-        routes = lib.concatMap (p: map (ip: { Destination = ip; }) p.allowedIPs) (lib.attrValues wg.peers);
+        routes = lib.concatMap (
+          p:
+          map (
+            ip:
+            let
+              sameFamily = lib.filter (s: lib.hasInfix ":" s == lib.hasInfix ":" ip) wg.sourceAddresses;
+            in
+            {
+              Destination = ip;
+            }
+            // lib.optionalAttrs (sameFamily != [ ]) { PreferredSource = builtins.head sameFamily; }
+          ) p.allowedIPs
+        ) (lib.attrValues wg.peers);
       }
     ) cfg.wireguard;
   };