Warn at configure time when NAX kernels are disabled - #3824

Fixes #3821.

What

Add a message(WARNING) in the else() branch of the NAX-kernel gate in
mlx/backend/metal/kernels/CMakeLists.txt, so a build that silently drops the
M-series Neural-Accelerator (NAX) kernels says so — and prints the two inputs
that decided it:

  else()
    message(
      WARNING "NAX kernels require Metal 4, macOS SDK >= 26.2, and "
              "MACOSX_DEPLOYMENT_TARGET >= 26.2 (SDK ${MACOS_SDK_VERSION}, "
              "CMAKE_OSX_DEPLOYMENT_TARGET=${CMAKE_OSX_DEPLOYMENT_TARGET}). "
              "Building without NAX kernels.")
    target_compile_definitions(mlx PRIVATE MLX_METAL_NO_NAX)
  endif()

Why

The NAX gate (added in #3622) requires Metal 4, macOS SDK ≥ 26.2, and
CMAKE_OSX_DEPLOYMENT_TARGET ≥ 26.2. CMake takes the deployment target from the
MACOSX_DEPLOYMENT_TARGET environment variable when the Python build toolchain
exports one (with it unset, the top-level CMakeLists falls back to the host OS
version). Any target in [14.0, 26.2) configures fine but silently compiles
without NAX — that is exactly how we hit it: a build with target 26.0 lost the
NAX kernels with no indication, costing ≈3–4× on prefill-dominated workloads
(NAX covers GEMM / quantized-matmul / attention) and a debugging session that
initially looked like an upstream performance regression. A configure-time
warning turns that silent cliff into a one-line diagnosis; printing the actual
SDK and deployment-target values makes it self-explanatory.

Notes on scope

• The warning fires on the condition (the gate evaluated false → NAX is
  being dropped), not on any inference about why the target is set the way it
  is. zcbenz noted on the issue he wasn't sure there is a reliable way to detect
  whether the target came from the Python toolchain vs. elsewhere — this change
  sidesteps that question entirely: it states the requirement, prints the
  observed values, and makes no claim about intent. Anyone who set the target
  deliberately can read it and move on.
• It also fires for hosts that cannot build NAX (older SDK / Xcode), where it
  is informational rather than actionable. If that is too chatty, the warning
  could be limited to the actionable case (SDK ≥ 26.2 but target < 26.2) with
  a message(STATUS) otherwise — either shape works; this PR keeps the simple
  form.
• JIT builds are unaffected (kernels are runtime-compiled and NAX availability
  is a runtime check there); non-macOS builds never enter this file.

+5 lines, cmake-format (v0.6.13, the pre-commit pin) clean, no behavior change
to the build output itself.