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/wireguard.nix

@@ -226,6 +226,8 @@ pkgs.testers.runNixOSTest {
               # No address on the tunnel: the site queries the LAN address and
               # is allowed because it routes 192.168.50.0/24 through the tunnel.
               dns = true;
+              # Router-originated traffic to the site uses the LAN address.
+              sourceAddresses = [ "192.168.10.1" ];
               peers.siteb = {
                 publicKey = key.siteb.pub;
                 endpoint = "203.0.113.60:51821";
@@ -386,6 +388,11 @@ pkgs.testers.runNixOSTest {
         siteb.succeed("ping -c 1 -W 2 -I 192.168.50.1 192.168.10.1")
         siteb.fail("ping -c 1 -W 2 -I 192.168.50.1 192.168.10.100")
 
+    with subtest("site-to-site: router-originated traffic uses the configured source"):
+        # Without it the kernel would pick an address by itself (the tunnel has none).
+        router.succeed("ip route get 192.168.50.1 | grep -q 'src 192.168.10.1'")
+        router.succeed("ping -c 1 -W 2 192.168.50.1")
+
     with subtest("site-to-site: the remote site can use the router's resolver"):
         siteb.succeed("dig +short +time=2 +tries=1 -b 192.168.50.1 @192.168.10.1 example.test | grep -qx 203.0.113.1")