feat(guests): place networked guests on tagged VLANs (task 0004)

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.

modules/network.nix

@@ -1,14 +1,16 @@
 {
   config,
   lib,
+  my,
   ...
 }:
 # The host networking foundation: per-VLAN bridges over a tagged trunk.
 let
   cfg = config.modules.network;
 
-  # Each tagged VLAN materializes as a bridge named for its id.
-  bridgeName = id: "br-vlan${toString id}";
+  # Each tagged VLAN materializes as a bridge named for its id, by the shared
+  # convention.
+  inherit (my) bridgeName;
 
   # The tagged sub-interface stacked on the trunk that feeds one bridge.
   vlanName = id: "${cfg.trunk}.${toString id}";