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.