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>

tests/assertions.nix

@@ -940,6 +940,45 @@ let
       };
       expect = [ "not a valid hostname" ];
     };
+    pppTransportValid = {
+      router = {
+        wan = {
+          ipv4 = {
+            type = "pppoe";
+            pppoe = {
+              vlan = 1001;
+              username = "u";
+              passwordFile = "/var/lib/pw";
+            };
+          };
+          ipv6 = {
+            enable = true;
+            transport = "ppp";
+          };
+        };
+        lans.main = lan {
+          interface = "eth0";
+          ipv6.pd = true;
+        };
+      };
+      expect = [ ];
+    };
+    pppTransportNeedsPppoe = {
+      router = {
+        wan = {
+          ipv4.type = "dhcp";
+          ipv6 = {
+            enable = true;
+            transport = "ppp";
+          };
+        };
+        lans.main = lan { interface = "eth0"; };
+      };
+      expect = [
+        "needs a PPPoE uplink"
+        "also used by a LAN"
+      ];
+    };
     nonOverlappingLans = {
       router.lans = {
         a = lan { };
@@ -988,6 +1027,7 @@ let
     "wgValid"
     "wanValid"
     "sharedLinkValid"
+    "pppTransportValid"
     "dnsValid"
   ];