Move the `router` service instance out of clan.nix into its own clan
module, inventory.nix, imported by clan.nix. Fleet-wide role settings
(mesh subnet, proxy domain/ACME) and the gw-cnx-1 site layout live there;
clan.nix keeps the machine list and the clan-core services. The gw-cnx-1
toplevel is byte-identical before and after the move.