Add current MLX arg-reduce native validation

[NripeshN]

Summary

• pin the current MLX Metal kernel tree at d9add9d11f3154111a4c85f267ec2fd307ecd18e and record its exact 42-unit / 17,478-entry zero-diagnostic census
• translate and package all 24 discovered arg_reduce.metal entries deterministically for Metal, OpenGL, and DirectX, pinning all 72 target artifacts
• require native macOS Metal, Linux glslang/SPIR-V, and Windows DXC compiler validation for all 24 entries while retaining numerical runtime/upstream-metallib parity for argmin_float32 and argmax_float32
• add fail-closed backend support for exact nested software-subgroup helper identities and two-word shuffle lowering in OpenGL, internal-only bfloat groupshared register representation in DirectX, and native-width narrow local arrays in Metal
• retain the initial current-tree aggregate-constructor, integral-reference pointer-offset, constrained free-function SFINAE, strict-JSON IEEE-754 input-token, and source-sensitive DXC native-16 repairs

Scope

This increment proves 24 / 17,478 entries for deterministic translation and native compiler validation, producing 72 pinned Metal/OpenGL/DirectX artifacts. Numerical runtime parity remains deliberately bounded to 2 / 17,478 entries: argmin_float32 and argmax_float32.

The compact contract explicitly does not claim complete-corpus translation, upstream MLX test-suite execution, or MLX host-runtime redirection yet.

Validation

• complete target harnesses: Metal 25 passed in 211.37s; OpenGL 25 passed in 219.68s; DirectX command-contract proof 25 passed in 218.83s
• Metal harness compiled and linked all 24 entries natively and executed the two float32 runtime/upstream-metallib parity entries
• OpenGL harness validated all 24 entries with glslangValidator and spirv-val; two-entry Mesa EGL runtime remains required in Linux CI
• DirectX harness reproduced all 24 artifacts through a strict exact-nine-argument shim requiring -enable-16bit-types, -WX, cs_6_6, and CSMain; real DXC/WARP remains required in Windows CI
• broad affected suites: 8,545 passed, 30 expected skips
• canonical Python 3.11 full suite: 18,436 passed, 32,528 expected skips, one known Hypothesis warning
• all-file pre-commit and warning-fatal Sphinx 8.2.3 passed
• support matrix is fresh; all 957 evidence rows are present with zero missing; CI coverage passed; 36-issue sync dry-run was non-mutating
• exact current-tree census: 42 units, 17,478 entries, zero diagnostics

Support issue traceability: no issue closed