router: optional Wi-Fi access point, enforce allowWan

wifi.* settings turn the gateway's own radios into the site AP (hostapd):
SSIDs are defined once in wifi.networks and act as untagged access ports of
their VLAN (each BSS joins br0 with the VLAN's PVID), radios pick what they
broadcast, passphrases are vars prompts. Extra SSIDs on a radio get BSSIDs
derived from its hardware address. A udev rule puts configured radios in AP
mode on appearance, since networkd cannot bridge a station-mode interface
and gives up before hostapd switches it. wifi.enable gates all of it.

The VM test grows a hwsim radio with two SSIDs and a WPA3 station in its
own netns that must get a Kea lease on the SSID's VLAN. Making the client
deterministic (route metrics, loose rp-filter, a guard against a vacuous
negative check) exposed that allowWan was never enforced: networking.nat
opens forward-to-WAN for all of its internalInterfaces, which listed every
VLAN. It now lists only the allowWan VLANs; the duplicate custom rule is
gone. No behavioural change on gw-cnx-1 (all its VLANs allow WAN).

modules/clan/router/firewall.nix

@@ -13,7 +13,6 @@ let
     lib.filterAttrs (_: vlan: vlan.allowWan) cfg.vlans
   );
   nonMgmtIfs = lib.filter (i: i != "vlan-mgmt") vlanIfs;
-  ifSet = ifs: "{ ${lib.concatStringsSep ", " (map (i: "\"${i}\"") ifs)} }";
 in
 {
   networking.nftables.enable = true;
@@ -44,13 +43,16 @@ in
     extraForwardRules = ''
       tcp flags syn tcp option maxseg size set rt mtu comment "MSS clamp for PPPoE mtu 1492"
       iifname "vlan-mgmt" accept comment "mgmt reaches all VLANs and the WAN"
-      iifname ${ifSet wanVlanIfs} oifname "ppp0" accept comment "LAN to internet"
     '';
   };
 
+  # networking.nat both masquerades and opens forward-to-WAN for exactly its
+  # internalInterfaces — so this list, not a rule of our own, is where
+  # `allowWan` is enforced. Listing every VLAN here would silently let
+  # allowWan = false VLANs out.
   networking.nat = {
     enable = true;
     externalInterface = "ppp0";
-    internalInterfaces = vlanIfs;
+    internalInterfaces = wanVlanIfs;
   };
 }