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>
