track cad/freecad/openscad modules referenced by t480

These modules define nixosModules.cad (and freecad/openscad) referenced by
the t480 host module list but were untracked, so the flake could not resolve
them (undefined variable 'cad'). Track them so the build evaluates.

modules/features/freecad/default.nix

@@ -0,0 +1,9 @@
+{...}: {
+  # Screenshot capture, region selection, annotation, and clipboard support for
+  # the Hyprland screenshot keybinds managed in the dotfiles repo.
+  flake.nixosModules.freecad = {pkgs, ...}: {
+    environment.systemPackages = with pkgs; [
+      freecad
+    ];
+  };
+}