Add doc on the required environment for Metal (#292)

Author: danieldk

README.md

@@ -73,6 +73,8 @@ See [dockerfiles/README.md](./dockerfiles/README.md) for more options, including
 
 - [Writing Hub kernels](./docs/writing-kernels.md)
 - [Building kernels with Nix](./docs/nix.md)
+- Framework-specific notes:
+  - [Metal](docs/metal.md)
 - [Building kernels with Docker](./docs/docker.md) (for systems without Nix)
 - [Local kernel development](docs/local-dev.md) (IDE integration)
 - [Kernel security](./docs/security.md)

docs/metal.md

@@ -0,0 +1,60 @@
+# Metal kernels 🤘
+
+Instructions on this page assume that you installed Nix with the
+[Determinate Nix installer](https://docs.determinate.systems/determinate-nix/).
+
+## Targeted macOS versions
+
+Since new macOS versions get [adopted quickly](https://telemetrydeck.com/survey/apple/macOS/versions/),
+we only support the latest major macOS version except for the first weeks
+after a release, when we also support the previous major version.
+
+We currently support macOS 26.0 and later on ARM64 (Apple silicon).
+
+## Requirements
+
+To build a Metal kernel, the following requirements must be met:
+
+- Xcode 26.x must be available on the build machine.
+- `xcode-select -p` must point to the Xcode 26 installation, typically
+  `/Applications/Xcode.app/Contents/Developer`. If this is not the case,
+  you can set the path with:
+  `sudo xcode-select -s /path/to/Xcode.app/Contents/Developer`
+- The Metal Toolchain must be installed. Starting with macOS 26, this is
+  a separate download from Xcode. You can install it with:
+  `xcodebuild -downloadComponent MetalToolchain`
+- The Nix sandbox should be set to `relaxed`, because the Nix derivation
+  that builds the kernel must have access to Xcode and the Metal Toolchain.
+  You can verify this by checking that `/etc/nix/nix.custom.conf` contains
+  the line:
+
+  ```
+  sandbox = relaxed
+  ```
+
+  If you had to add the line, make sure to restart the Nix daemon:
+
+  ```
+  sudo launchctl kickstart -k system/systems.determinate.nix-daemon
+  ```
+
+You can check these requirements as follows. First, you can check the Xcode
+version as follows:
+
+```bash
+$ xcodebuild -version
+Xcode 26.1
+Build version 17B55
+```
+
+The reported version must be 26.0 or newer. Then you can validate that the
+Metal Toolchain is installed with:
+
+```bash
+$ xcodebuild -showComponent metalToolchain
+Asset Path: /System/Library/AssetsV2/com_apple_MobileAsset_MetalToolchain/68d8db6212b48d387d071ff7b905df796658e713.asset/AssetData
+Build Version: 17B54
+Status: installed
+Toolchain Identifier: com.apple.dt.toolchain.Metal.32023
+Toolchain Search Path: /private/var/run/com.apple.security.cryptexd/mnt/com.apple.MobileAsset.MetalToolchain-v17.2.54.0.mDxgz0
+```