Add VLANs, firewall zones and inter-zone rules

LANs can sit on an 802.1Q VLAN (`vlan` on a trunk `interface`); networkd
creates the device and brings the trunk up. LANs belong to a named `zone`
(default: their own name). Traffic inside a zone is forwarded freely, traffic
between zones is denied unless a `router.firewall.rules` entry allows it
(protocol, ports, optional destination inside the target zone).

Internet access is now opt-in per LAN (`internet`, default false), replacing
the previous allow-out default. Assertions reject unknown zones, bad
destinations, duplicate devices, over-long device names and a WAN trunk.

Kea subnet IDs now come from the network address so adding LANs does not
renumber them and orphan persistent leases.

Tests: a five-VM `vlans` check (untagged + two VLANs, shared zone, one-way
rules, per-host destination restriction), new assertion cases, parseNet unit
tests, and shared test tooling in tests/common.nix.

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

PLAN.md

@@ -62,6 +62,8 @@ examples/    # sample configs
 
 ## Status
 
+**VLANs + zones done**: `vlans` VM test (5 VMs: ISP, router, untagged `main`, VLAN 20 `iot`, VLAN 30 `cams`) covers VLAN devices, per-VLAN DHCP/gateway, opt-in internet, DNS vs SSH per LAN, intra-zone forwarding across VLANs, one-directional inter-zone rules and the destination restriction (each mutation-tested). Not covered yet: IPv6 on VLAN LANs (only the PD LAN has IPv6), port forwards from the WAN.
+
 **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.
@@ -75,7 +77,7 @@ 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` and `pppoe` (VM tests), `lib` (helper unit tests), `assertions` (broken configs must fail with exact messages). Not yet covered: VLANs, WireGuard, formal zones.
+- Checks: `basic`, `pppoe` and `vlans` (VM tests), `lib` (helper unit tests), `assertions` (broken configs must fail with exact messages). Not yet covered: WireGuard, port forwards, WAN-originated rules.
 
 ## IPv6 design (confirmed)
 
@@ -93,6 +95,14 @@ Learned along the way:
 - 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.
 
+## VLANs and zones (confirmed)
+
+- **VLAN devices** are created by the router: `router.lans.<name> = { interface = <trunk>; vlan = <id>; }` makes `<trunk>.<id>` via networkd. A trunk may also carry one untagged LAN. Assertions: duplicate device, device name > 15 chars, WAN interface reused as trunk.
+- **Zones** are an attribute on the LAN (`zone`, default = the LAN's own name); LANs sharing a zone may forward to each other, traffic **between zones is denied** unless a rule in `router.firewall.rules` allows it (`from`, `to`, `protocol`, `ports`, optional IPv4 `destination` that must lie inside a LAN of the destination zone). Rules are one-directional; return traffic via conntrack. Assertions reject unknown zones, from == to, ports without tcp/udp, bad/out-of-zone/host-bit destinations.
+- **Internet is opt-in per LAN** (`internet`, default false) — this replaces the earlier "LAN allow-out" default; any LAN that should reach the WAN must set it.
+- Router services stay per LAN: DNS and DHCP always, SSH only with `allowManagement`.
+- Kea subnet IDs are derived from the network address, so adding or removing LANs does not shift IDs (and orphan persistent leases).
+
 ## Open items
 
 - **Web UI** (stats and possibly configuration) — to be discussed later. Considerations: read-only stats first is cheap; config editing must write declarative config (Nix/JSON) that feeds the module, not mutate the live system.