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>

tests/assertions.nix

@@ -999,6 +999,40 @@ let
       router.lans.main = lan { ipv6.address = "192.168.1.1/24"; };
       expect = [ "not an IPv6 address in CIDR" ];
     };
+    wgSourceOk = {
+      router = {
+        lans.main = lan { };
+        wireguard.s2s = wg { sourceAddresses = [ "192.168.10.1" ]; };
+      };
+      expect = [ ];
+    };
+    wgSourceTwoV4 = {
+      router = {
+        lans.main = lan { };
+        wireguard.s2s = wg {
+          addresses = [ "10.100.0.1/24" ];
+          sourceAddresses = [
+            "192.168.10.1"
+            "10.100.0.1"
+          ];
+        };
+      };
+      expect = [ "at most one address per family" ];
+    };
+    wgSourceForeign = {
+      router = {
+        lans.main = lan { };
+        wireguard.s2s = wg { sourceAddresses = [ "192.168.99.9" ]; };
+      };
+      expect = [ "must be the router's address on a LAN or a tunnel" ];
+    };
+    wgSourceGarbage = {
+      router = {
+        lans.main = lan { };
+        wireguard.s2s = wg { sourceAddresses = [ "nonsense" ]; };
+      };
+      expect = [ "sourceAddresses must be IPv4/IPv6 addresses" ];
+    };
     nonOverlappingLans = {
       router.lans = {
         a = lan { };
@@ -1049,6 +1083,7 @@ let
     "sharedLinkValid"
     "pppTransportValid"
     "ulaValid"
+    "wgSourceOk"
     "dnsValid"
   ];