Bind new_thread_unsafe_stream

[CharlieTLe]

Summary

Expose mlx's new_thread_unsafe_stream as mlx_stream_new_thread_unsafe.

Streams became thread affine in mlx 0.31.2 (#3348, #3281): a stream's GPU command encoder is registered in a thread_local map, so evaluating on a stream from a thread other than the one that created it raises There is no Stream(gpu, N) in current thread.

mlx added new_thread_unsafe_stream in ml-explore/mlx#3578 specifically for this case:

This provides a way for language bindings (like Swift) to evaluate graph asynchronously via threads, on the condition that they add locks on themselves.

mlx-c does not bind it, so those bindings can't reach it. mlx-swift currently carries a local C shim to work around exactly this, which this PR would let it delete.

Before / after

examples/example-thread-stream.c creates a stream on the main thread and evaluates on it from a worker thread. With mlx_stream_new_device:

MLX error: There is no Stream(gpu, 0) in current thread. at mlx/c/array.cpp:443

With mlx_stream_new_thread_unsafe:

2 + 3 evaluated on another thread = 5

Note on the mlx pin

new_thread_unsafe_stream only exists in mlx 0.32.0, so GIT_TAG moves from v0.31.2 to v0.32.0. I kept that to the one-line bump rather than regenerating bindings for 0.32.0, on the assumption you'd rather do that as its own change — happy to drop the bump if you'd prefer to land this after a 0.32.0 regen.

The existing bindings compile unchanged against 0.32.0, and example, example-grad, example-closure, example-graph and example-export all pass.

Naming

I went with mlx_stream_new_thread_unsafe to sit alongside mlx_stream_new_device in the mlx_stream_new* constructor family. Happy to rename to mlx_new_thread_unsafe_stream (closer to the C++ name) or anything else you prefer.

Possible follow-up

new_thread_local_stream / stream_from_thread_local_stream / clear_streams are also unbound. They need a new opaque mlx_thread_local_stream type with its own private accessors, so I left them out to keep this focused — glad to do it in a separate PR if it's wanted.

Checklist

• Formatted with clang-format using the repo's .clang-format
• Added an example exercising the new API
• Built and ran locally (macOS 27.0, Apple Silicon)