nix: support unified apple-sdk

.devops/nix/package.nix

@@ -13,6 +13,7 @@
   cudaPackages,
   autoAddDriverRunpath,
   darwin,
+  apple-sdk ? null,
   rocmPackages,
   vulkan-headers,
   vulkan-loader,
@@ -76,15 +77,17 @@ let
   '';
 
   # apple_sdk is supposed to choose sane defaults, no need to handle isAarch64
-  # separately
+  # separately.
+  # Modern Nixpkgs (25.05+) provides a unified 'apple-sdk' derivation.
+  # Legacy Nixpkgs (24.05/24.11) uses 'darwin.apple_sdk.frameworks'.
   darwinBuildInputs =
-    with darwin.apple_sdk.frameworks;
-    [
-      Accelerate
-      CoreVideo
-      CoreGraphics
-    ]
-    ++ optionals useMetalKit [ MetalKit ];
+    if apple-sdk != null then
+      # Unified SDK: single package provides all frameworks
+      [ apple-sdk ]
+    else
+      with darwin.apple_sdk.frameworks;
+      [ Accelerate CoreVideo CoreGraphics ]
+      ++ lib.optional useMetalKit MetalKit;
 
   cudaBuildInputs = with cudaPackages; [
     cuda_cudart