refactor: rename the three VM hosts to 12kingdoms-*

Drop the now-meaningless kubevirt- prefix and move the hosts out of
hosts/k8s/, following the existing 12kingdoms-shoukei layout: the
directory and output carry the theme prefix while the configuration,
hostname and colmena target are the bare name (shoryu, shushou, youko).

- hosts/k8s/kubevirt-* -> hosts/12kingdoms-*; the shared disko config
  becomes host-disko-fs.nix; the now-different relative imports are fixed
- outputs, vars/networking.nix keys, the Justfile recipes/tags, the eval
  tests and the docs follow
- colmena reaches these hosts by IP (a new targetHost override), since
  their DNS name changes with the hostname and is not resolvable until
  the router learns it

lib/colmenaSystem.nix

@@ -8,6 +8,9 @@
   system,
   tags,
   ssh-user,
+  # optional override; defaults to the host name. Use an IP when the host's
+  # DNS name is not resolvable yet (e.g. right after a hostname change).
+  targetHost ? null,
   genSpecialArgs,
   specialArgs ? (genSpecialArgs system),
   ...
@@ -20,7 +23,7 @@ in
   deployment = {
     inherit tags;
     targetUser = ssh-user;
-    targetHost = name; # hostName or IP address
+    targetHost = if targetHost != null then targetHost else name;
   };
 
   imports =