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

Justfile

@@ -197,7 +197,7 @@ upload-vm name size verbosity="normal":
   use {{utils_nu}} *;
   upload-vm {{name}} {{size}} {{verbosity}}
 
-# Deploy all the KubeVirt nodes(Physical machines running KubeVirt)
+# Deploy all the VM hosts (physical machines running the VMs)
 [linux]
 [group('homelab')]
 lab mode="switch":
@@ -206,17 +206,17 @@ lab mode="switch":
 [linux]
 [group('homelab')]
 shoryu mode="switch":
-  colmena apply {{mode}} --on '@kubevirt-shoryu' --verbose --show-trace
+  colmena apply {{mode}} --on '@shoryu' --verbose --show-trace
 
 [linux]
 [group('homelab')]
 shushou mode="switch":
-  colmena apply {{mode}} --on '@kubevirt-shushou' --verbose --show-trace
+  colmena apply {{mode}} --on '@shushou' --verbose --show-trace
 
 [linux]
 [group('homelab')]
 youko mode="switch":
-  colmena apply {{mode}} --on '@kubevirt-youko' --verbose --show-trace
+  colmena apply {{mode}} --on '@youko' --verbose --show-trace
 
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