Add Metal DLPack zero-copy sharing

[XXXXRT666]

Proposed changes

This draft adds zero-copy Metal DLPack sharing for MLX arrays and PyTorch MPS tensors.

This PR depends on #3495 and requires nanobind support.

The main changes are:

• Import Metal DLPack arrays by wrapping the underlying Metal buffer instead of copying through CPU.
• Export MLX arrays to Metal DLPack using the MLX Metal buffer and DLPack byte_offset.
• Add mx.from_dlpack(..., copy=...) controls for Metal DLPack inputs.
• Keep mx.array(...) zero-copy for Metal DLPack inputs unless an explicit different dtype is requested.
• Document the explicit synchronization requirements between PyTorch MPS and MLX.

The shared lifetime is tied to the exported or imported buffer. Synchronization remains explicit: PyTorch writes require torch.mps.synchronize() before MLX reads, and MLX writes require mx.eval(...) before PyTorch reads.

For MLX arrays exported to PyTorch, later MLX updates may rebind the MLX array to a new buffer while the PyTorch tensor continues to reference the exported buffer.

Checklist

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• 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)