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>

PLAN.md

@@ -62,6 +62,10 @@ examples/    # sample configs
 
 ## Status
 
+**PPPoE done**: `pppoe` VM test = the same topology with a fake ISP running `pppoe-server` (VLAN 7, CHAP, password containing a quote) — covers session, NAT, MTU, MSS clamp (mutation-tested) and WAN-deny. `basic` remains the DHCPv4 variant.
+
+**IPv6 + prefix delegation done (single /64)**: `router.wan.ipv6.enable` and `router.lans.<name>.ipv6.pd` (at most one LAN, asserted). The VM test now covers the router's DHCPv6 address and delegated /64, SLAAC on the client, routing without NAT66, and WAN-deny over IPv6 (mutation-tested). Still open: RDNSS, other LANs are IPv4-only, DHCPv6 reservations.
+
 **Phase 1 (MVP) skeleton done**: flake, `router.*` schema (wan, lans, dns.forwarders), networkd, nftables (zone-less for now: WAN/LAN), Kea DHCPv4, Unbound, assertions. `nix build .#checks.x86_64-linux.basic` runs a 3-node VM test (upstream/router/client) covering WAN DHCP, LAN DHCP+DNS, NAT, WAN-deny and LAN SSH.
 
 Interactive lab: `nix run .#lab` (x86_64) opens tmux with one console pane per VM plus the test-driver REPL (`router.shell_interact()`, `client.succeed(...)`, `test_script()`). The REPL pane also shows all VM output interleaved.
@@ -71,7 +75,23 @@ Learned along the way:
 - In NixOS tests `fail(...)` also passes on "command not found": install probe tools explicitly and add a positive control per negative check.
 - Unbound answers `.test` locally (RFC 6761) and validates DNSSEC; tests using fake domains need both `local-zone ... transparent` and `domain-insecure` (both verified necessary; set in the test, not the module).
 - `modules/router/lib.nix` has IPv4 helpers (parse/format/netmask/cidr/overlap). `ipv4.subnet` is now derived from `address` + `prefixLength` (read-only). Assertions also cover pool bounds, pool containing the router address, and overlapping LANs. IPv6 helpers (prefix arithmetic for PD subnet-IDs) are still to do.
-- Checks: `basic` (VM test), `lib` (helper unit tests), `assertions` (broken configs must fail with exact messages). Not yet covered: IPv6, VLANs, WireGuard, formal zones.
+- Checks: `basic` and `pppoe` (VM tests), `lib` (helper unit tests), `assertions` (broken configs must fail with exact messages). Not yet covered: VLANs, WireGuard, formal zones.
+
+## IPv6 design (confirmed)
+
+- **WAN**: mirrors the existing router's uplink config — networkd `DHCP` (v6), `IPv6AcceptRA = true`, `KeepConfiguration = yes`, `dhcpV6Config = { WithoutRA = "solicit"; UseDelegatedPrefix = true; }`. The ISP sends RAs (default route) without DHCPv6 flags, so the router solicits on its own.
+- **Delegation**: the ISP delegates a single **/64**, which cannot be split. Exactly one LAN may claim it via an explicit flag (`router.lans.<name>.ipv6.pd = true`); an assertion rejects more than one.
+- **LANs**: SLAAC only (RA, no DHCPv6). All other LANs/VLANs are **IPv4-only** for now (ULA or NPTv6 can come later).
+- **Firewall**: inet table covers v6; WAN→LAN forwarding stays denied, no NAT66.
+- **Test**: fake ISP on `upstream` = Kea DHCPv6 (IA_NA + IA_PD /64 from a /48 pool) + networkd RA sender (default route only) + a static route for the delegated prefix back to the router.
+- **Known gap**: no RDNSS in RAs yet (the router's global LAN address is not known at eval time); clients still get DNS via DHCPv4.
+
+## PPPoE uplink (confirmed)
+
+- **IPv4 via PPPoE** (`router.wan.ipv4.type = "pppoe"`), carried on an optional tagged **VLAN** on the WAN link (`pppoe.vlan`); native DHCPv6/PD stays on the untagged link.
+- `wan.interface` is the physical link; the derived `wan.ipv4.interface` (`pppoe-wan` or the link) is where NAT, MSS clamping and the IPv4 forwarding rule apply. IPv6 forwarding keeps using the physical link.
+- pppd (`pppoe.so`) runs as `router-pppoe.service`, ordered after the VLAN device. Credentials: `username` inline, password from `pppoe.passwordFile` (type forbids Nix-store paths); the options file is generated into a root-only runtime dir. **Deployment must provide that file** on persistent storage (or via agenix/sops).
+- **MTU 1492 + TCP MSS clamp** (1452) in both directions on the PPP device; RFC 4638 not implemented.
 
 ## Open items