A guest sets `vlan` to attach to its host's `br-vlan<id>` bridge, `mac`
to reuse an existing address (else a stable one is derived from its
namespace path and read back via `nix eval`), and `address` for a static
IP (else DHCP). The MAC and address are pinned inside the guest by its
own networkd, the only stable MAC pin for a nested container. A guest
naming a VLAN its host has not declared fails the build with an
actionable message.
The `br-vlan<id>` naming moves into a shared `bridgeName` in the lib, so
the bridge a guest attaches to and the bridge the host emits have one
source.
