Fix metallib path for cargo install — use stable ~/.mlx/ directory

[rgbkrk]

Problem

When a binary using mlx-rs is installed via cargo install, it fails at runtime:

MLX error: Failed to load the default metallib. library not found library not found library not found

cargo install builds in a temporary directory, then copies only the binary to ~/.cargo/bin/. The CMake build bakes METAL_PATH as an absolute path to the metallib inside this temp dir. After install, the temp dir is deleted and the metallib is unreachable.

Fix

Override MLX_METAL_PATH in the CMake config to point to a stable versioned directory: ~/.mlx/lib/v{mlx-sys-version}/. The directory is created before CMake runs so CMake outputs the compiled metallib there directly. The compiled-in METAL_PATH now survives temp dir cleanup.

MLX's runtime metallib search order:

1. Co-located with binary → still works for local builds
2. Bundle resources → still works
3. SwiftPM bundle → still works
4. METAL_PATH compiled-in fallback → now points to ~/.mlx/lib/v0.2.0/mlx.metallib ✅

Testing

Tested with the voice TTS crate:

# Patched build — metallib goes to stable location
cargo build --release -p voice
strings target/release/voice | grep metallib
# Shows: /Users/.../.mlx/lib/v0.2.0/mlx.metallib

ls ~/.mlx/lib/v0.2.0/mlx.metallib
# -rw-r--r--  84M  mlx.metallib

# Install and run — works even though build dir is gone
cargo install --path crates/voice-cli --force
voice -o /tmp/test.wav "hello"  # Success!

Fixes #327

Note: this PR is based on the v0.25.3 tag. Happy to rebase onto main if preferred — the build.rs on main has additional changes (clang_rt workaround) but the fix is the same.

[minghuaw]

Would you please rebase onto the main branch? I'll try to take a look this weekend.

[rgbkrk]

done