Add IPv6 prefix delegation and PPPoE uplink

IPv6: router.wan.ipv6.enable mirrors the existing uplink config (DHCPv6 with
WithoutRA=solicit, delegated prefix). The ISP delegates a single /64, so
exactly one LAN may claim it via router.lans.<name>.ipv6.pd (asserted); other
LANs stay IPv4-only. SLAAC only, no NAT66.

PPPoE: router.wan.ipv4.type = "pppoe" runs pppd on an optional VLAN of the
uplink, with the password read from a file at service start (never the Nix
store). NAT, forwarding and MSS clamping (MTU 1492) use the derived PPP
device; native IPv6 stays on the untagged link.

Tests: basic.nix is parameterised by uplink type; the pppoe variant uses a
fake ISP with pppoe-server (VLAN 7, CHAP). Both variants cover IPv6 PD, SLAAC,
routing without NAT and WAN-deny. Assertion tests gain PD/PPPoE cases and a
full-system evaluation step.

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

modules/router/dhcp.nix

@@ -25,8 +25,14 @@ in
           interface = l.interface;
           pools = [ { pool = "${l.dhcp.pool.start} - ${l.dhcp.pool.end}"; } ];
           option-data = [
-            { name = "routers"; data = l.ipv4.address; }
-            { name = "domain-name-servers"; data = l.ipv4.address; }
+            {
+              name = "routers";
+              data = l.ipv4.address;
+            }
+            {
+              name = "domain-name-servers";
+              data = l.ipv4.address;
+            }
           ];
         }) dhcpLans;
       };