Add an MLX Array API compatibility namespace

[declanhealy2]

Summary

This adds a first-class array_api_compat.mlx namespace around the native mlx.core.array type.

The implementation deliberately follows the ownership boundaries discussed in data-apis#450:

• small signature, keyword, dtype, and return-container adaptations live here;
• MLX execution remains native;
• no array subclassing or monkeypatching;
• no hidden NumPy fallback;
• no fabricated per-array device registry;
• intrinsic data-dependent-shape limitations are documented as explicit conformance xfails.

Included

• namespace discovery through array_namespace() and MLX inspection helpers;
• creation, manipulation, reduction, searching, sorting, FFT, and linear-algebra adapters;
• __array_namespace_info__() for the compat namespace;
• focused MLX tests;
• a macOS workflow running both the focused tests and the full Array API conformance suite;
• explicit known-failure files rather than blanket skips.

Validation

The branch includes a dedicated macOS CI workflow. This PR is intentionally draft while that full conformance run is being used to refine the adapters and the known intrinsic limitations.