fix: fail build when Metal compiler header resolution fails

[dogukanveziroglu]

Summary

make_compiled_preamble.sh silently produces broken JIT Metal kernel sources when xcrun -sdk macosx metal fails during the build (e.g. due to misconfigured Xcode CLI tools or missing SDK path).

The script uses xcrun metal -H to resolve header dependencies, but never checks whether the command succeeded. When it fails, the error message words (e.g. "error:", "to", "not", "utility", "or",
"developer", "PATH") are parsed as header file names. This causes critical headers like bf16.h (which defines typedef bfloat bfloat16_t;) to be silently omitted from the embedded Metal kernel sources
in libmlx.dylib.

The build completes successfully, but at runtime any operation requiring JIT Metal compilation (e.g. gather_front on bfloat16 tensors) fails with:

RuntimeError: [metal::Device] Unable to build metal library from source
mlx/backend/metal/kernels/utils.h: error: unknown type name 'bfloat16_t'

This is extremely difficult to diagnose because:

• The build reports no errors
• Simple operations (using precompiled metallib kernels) work fine
• The error only appears at runtime when a JIT-compiled kernel is needed
• The root cause (missing bf16.h in the embedded source) is not visible from the error message

Fix

Add validation after the xcrun metal -H call to check that the output contains valid header dependency lines (starting with . characters). If invalid lines are detected, the build fails immediately with a
clear error message pointing to the Metal toolchain configuration.

Checklist

• I have read the CONTRIBUTING document
• I have run pre-commit run --all-files to format my code / installed pre-commit prior to committing changes
• I have added tests that prove my fix is effective or that my feature works
• I have updated the necessary documentation (if needed)

No test added — there is no existing test infrastructure for build shell scripts. The fix is a straightforward validation guard that fails early instead of silently continuing.

[zcbenz]

This is a very useful improvement, thanks!