kayla: add kayla-vm variant for trying the Plasma session in a window

flake.nix

@@ -57,6 +57,11 @@
         # bring-up (arr-interconnect especially) in a throwaway VM:
         #   nixos-rebuild build-vm --flake .#mediaserver-vm
         mediaserver-vm     = mkHost "FredOS-Mediaserver" [ ./hosts/hardware/vm.nix ];
+
+        # Kayla's desktop in a window, for trying the Plasma session out before
+        # her machine exists.
+        #   nixos-rebuild build-vm --flake .#kayla-vm
+        kayla-vm           = mkHost "FredOS-Kayla"       [ ./hosts/hardware/kayla-vm.nix ];
       };
     };
 }