feat(blender): optional CUDA build

Off by default: it is only useful on an NVIDIA host, and no binary cache
serves it (cuda-maintainers was measured to change nothing), so enabling
it rebuilds Blender, OpenUSD, OpenSubdiv and OpenImageDenoise.

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

home/common/programs/3d.nix

@@ -1,11 +1,33 @@
 {
+  config,
+  lib,
   pkgs,
   ...
 }:
+let
+  cfg = config.custom.blender;
+in
 {
-  home.packages = with pkgs; [
+  options.custom.blender = {
-    blender
+    cuda = lib.mkOption {
-    prusa-slicer
+      type = lib.types.bool;
+      default = false;
+      description = ''
+        Build Blender with CUDA/OptiX so Cycles renders on the GPU instead of
+        the CPU. Only useful on an NVIDIA host; enabling it on arrakis (Intel)
+        would mean a long build for nothing.
+
+        No binary cache serves this — cuda-maintainers was measured to make no
+        difference — so flipping this on rebuilds Blender, OpenUSD, OpenSubdiv
+        and OpenImageDenoise from source.
+      '';
+    };
+  };
+
+  config = {
+    home.packages = [
+      (if cfg.cuda then pkgs.blender.override { cudaSupport = true; } else pkgs.blender)
+      pkgs.prusa-slicer
     ];
 
     fonts.fontconfig.enable = true;
@@ -17,4 +39,5 @@
       noDisplay = true;
       mimeType = [ "x-scheme-handler/prusaslicer" ];
     };
+  };
 }