Add mlx_metal_set_metallib_path() C API

[olilarkin]

What

Exposes the new mlx::core::metal::set_metallib_path() through the C interface as mlx_metal_set_metallib_path(const char* path), following the existing error-handling convention of the other mlx_metal_* wrappers.

Why

Required so Swift / other C consumers can set a custom mlx.metallib path. See the use case in the tracking issue.

Depends on

• Add metal::set_metallib_path() to override the metallib search path mlx#3597 (the underlying C++ function). The mlx submodule here will need a bump to the merged commit once that lands.

Companion PRs

Part of a three-PR chain (C++ → C → Swift):

1. Add metal::set_metallib_path() to override the metallib search path mlx#3597 — C++ set_metallib_path
2. Add mlx_metal_set_metallib_path() C API #117 — this PR (C API wrapper)
3. Add GPU.setMetallibPath() for plugin bundle support mlx-swift#416 — Swift GPU.setMetallibPath()

Tracking issue: ml-explore/mlx-swift#415.

[RNT56]

I looked at this while working on the SwiftPM metallib resource PR in mlx-swift.

This is the API I need there. I made a small branch on my fork that rebases this on current mlx-c main and points the MLX dependency at the merged mlx#3597 commit:

RNT56/mlx-c:upstream-pr/metallib-path-c-api

It builds for me with:

• cmake -S . -B build/metallib-path-api -DMLX_C_BUILD_EXAMPLES=OFF -DCMAKE_BUILD_TYPE=Release
• cmake --build build/metallib-path-api -j2

No need to use my branch directly if you prefer updating this PR here. I just wanted to share it in case it helps move the chain forward.