Gate NAX JIT sources by macOS SDK version

[ausimian]

Proposed changes

When MLX_METAL_JIT=ON, backend/metal/CMakeLists.txt unconditionally invokes make_jit_source() on the NAX kernel sources. Those headers transitively #include <MetalPerformancePrimitives/MetalPerformancePrimitives.h>, which only ships with the macOS 26.2+ SDK, so JIT builds currently fail on older SDKs (e.g. Xcode 15.x on GitHub-hosted macos-14 runners):

fatal error: 'MetalPerformancePrimitives/MetalPerformancePrimitives.h' file not found
  included from vendor/mlx/mlx/backend/metal/kernels/steel/gemm/nax.h:12

The AOT branch in kernels/CMakeLists.txt already gates NAX by the same SDK check (MLX_METAL_VERSION GREATER_EQUAL 400 AND MACOS_SDK_VERSION GREATER_EQUAL 26.2). This PR mirrors that gate in the JIT branch:

• When the SDK gate passes: no behavioural change — NAX JIT sources are emitted as before.
• When it fails: define MLX_METAL_NO_NAX (so is_nax_available() returns false at runtime) and emit a tiny generated nax_stubs.cpp with empty-string stubs for the NAX JIT source providers (metal::gemm_nax, metal::quantized_nax, …). The stubs are needed only so jit_kernels.cpp still links; they are never exercised at runtime because the NAX code paths are guarded by is_nax_available().

On older SDKs, MLX_METAL_JIT=ON now produces the same no-NAX binary that MLX_METAL_JIT=OFF already does — it just uses the JIT path for the non-NAX kernels.

Context

Discovered while plumbing MLX_METAL_JIT=ON through a downstream consumer's CI (GitHub-hosted macos-14 / Xcode 15.x). MLX's own CI builds JIT on a self-hosted runner with Xcode 26, so the upstream CI does not currently exercise the JIT path on older SDKs.

Tested

• Local build on macOS 26.4 / Xcode 26 — SDK gate passes, all NAX JIT sources emitted, behaviour identical to current main.
• Build + run the downstream consumer's full test suite with MLX_METAL_JIT=ON on GitHub-hosted macos-14 — previously broken at the nax.h preprocess step, now links and passes after this patch.

Checklist

• 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 — CMake-only change; happy to run cmake-format if the maintainers prefer a specific output.
• I have added tests that prove my fix is effective or that my feature works — no test changes; this is a build-system guard mirroring an existing one.
• I have updated the necessary documentation (if needed) — no public doc change.