Add IPv6 inside the PPP session and use enp2s0 in the example

- wan.ipv6.transport = "ppp": IPv6CP in pppd, router advertisements and
  DHCPv6 prefix delegation on the PPP device, DHCPv6 accepted on the PPP
  device, IPv6 MSS clamped to MTU-60. Allowed on a shared single-NIC
  link. A build-time check requires a PPPoE uplink.
- IPv4 MSS clamping now only matches IPv4 packets.
- The example uses the new box's NIC name (enp2s0).

Covered by the onestick VM test (radvd and a minimal DHCPv6-PD
responder as the ISP) and assertion cases.

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

modules/router/options.nix

@@ -71,10 +71,27 @@ in
         };
       };
 
-      ipv6.enable = mkEnableOption ''
-        IPv6 on the uplink: DHCPv6 (soliciting even without RA flags) plus
-        prefix delegation. The ISP is expected to delegate a single /64
-      '';
+      ipv6 = {
+        enable = mkEnableOption ''
+          IPv6 on the uplink: DHCPv6 (soliciting even without RA flags) plus
+          prefix delegation. The ISP is expected to delegate a single /64
+        '';
+
+        transport = mkOption {
+          type = types.enum [
+            "native"
+            "ppp"
+          ];
+          default = "native";
+          description = ''
+            Where IPv6 runs. `native`: untagged on the uplink link, next to a
+            DHCPv4 or PPPoE (tagged) IPv4 uplink. `ppp`: inside the PPPoE
+            session (IPv6CP), so DHCPv6 and router advertisements arrive on
+            the PPP device; needs a PPPoE uplink, and leaves the physical link
+            free for a LAN.
+          '';
+        };
+      };
     };
 
     lans = mkOption {