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/pppoe.nix

@@ -28,7 +28,12 @@ let
     defaultroute
     replacedefaultroute
     noipdefault
-    nodefaultroute6
+    ${
+      if wan.ipv6.enable && wan.ipv6.transport == "ppp" then
+        "+ipv6\n    ipv6cp-use-ipaddr"
+      else
+        "nodefaultroute6"
+    }
     noccp
     mtu ${toString pppoe.mtu}
     mru ${toString pppoe.mtu}