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

hosts/hardware/kayla-vm.nix

@@ -0,0 +1,24 @@
+#./hosts/hardware/kayla-vm.nix
+# Stand-in "hardware" for `nixos-rebuild build-vm --flake .#kayla-vm`.
+#
+# Keeps hostName = FredOS-Kayla so hosts/FredOS-Kayla.nix switches on and you
+# get the real Plasma session, stylix dark colours and app set — just to see
+# what it feels like before committing real hardware to it. qemu-vm.nix
+# supplies "/" and the bootloader, so no fileSystems here.
+{ lib, ... }:
+{
+  networking.hostName = "FredOS-Kayla";
+  system.stateVersion = "25.11";
+  nixpkgs.hostPlatform = "x86_64-linux";
+
+  # The real user has no password until `passwd kayla` on first boot, which
+  # would leave the VM stuck at SDDM. VM-only, hence not in settings/users.nix.
+  users.users.kayla.initialPassword = "kayla";
+
+  virtualisation.vmVariant.virtualisation = {
+    memorySize = 8192;
+    cores = 4;
+    diskSize = 8192;   # host store is mounted in, this is just /home + state
+    graphics = true;
+  };
+}