Add k3s-single-node module with GPU/CDI support

Provides a NixOS module for running a single-node k3s cluster with
NVIDIA GPU support via CDI (Container Device Interface). Includes
automatic deployment of the generic-cdi-plugin DaemonSet for GPU
resource allocation.

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

nixos/configuration.nix

@@ -21,6 +21,7 @@
     ./imalison.nix
     ./internet-computer.nix
     ./k3s.nix
+    ./k3s-single-node.nix
     ./kat.nix
     ./keybase.nix
     ./kubelet.nix