Add mlx_array_detach to break graph links before free

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Summary

Adds mlx_array_detach() to the C API, wrapping the existing mlx::core::array::detach() method.

When releasing model weights or other arrays that may still be linked in the computation graph (e.g. sibling outputs from a traced/compiled forward pass), calling mlx_array_free() alone can leave dangling references in shared ArrayDesc nodes. Detaching first clears those sibling links so the buffer can be freed safely.

• Declares mlx_array_detach in mlx/c/array.h with documentation
• Implements a thin wrapper in mlx/c/array.cpp
• No-op when arr.ctx is null, consistent with mlx_array_free_

Motivation

Downstream bindings (Go via dynamic libmlxc, Swift, etc.) that hold weight arrays across inference sessions currently need to patch array.cpp locally or call into C++ internals to detach before free. Exposing this in the public C API removes that friction.

Test plan

• Build mlx-c against MLX v0.31.2
• Verify mlx_array_detach is exported from libmlxc
• Manual: create an array via ops, detach, free — no crash
• Manual: detach + free weight arrays after a compiled forward pass — no use-after-free

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Test plan results (macOS arm64)

Verified locally on feat/array-detach (3da2b0a):

• Build mlx-c against MLX v0.31.2 — cmake -B build-static -DCMAKE_BUILD_TYPE=Release fetched MLX v0.31.2; cmake --build build-static --target mlxc succeeded (static lib).
• Verify mlx_array_detach exported from libmlxc — nm build-static/libmlxc.a | rg mlx_array_detach → T _mlx_array_detach.
• Manual: ops → detach → free — add on GPU, detach operands + result + empty array, free; no crash.
• Manual: compiled forward → detach weight → free — mlx_compile + matmul closure using a weight array captured via payload; detach + free weight after apply; no crash/use-after-free.

MLX version: 0.31.2
PASS: create array via ops, detach, free
PASS: detach + free weight after compiled forward pass
All mlx_array_detach tests passed.

Note: BUILD_SHARED_LIBS=ON failed to link libjaccl.dylib on this host (missing C++ stdlib symbols); static libmlxc.a build and symbol export check above are unaffected.