Add atomic memory orderings for Metal

[VEZY]

Note: I used Codex with GPT-5.5 High to implement this feature (both on Metal.jl and here). The original issue for context is linked to building a TLAS on GPU with RayCore.jl (JuliaGeometry/Raycore.jl#19). The PR on Metal is here: JuliaGPU/Metal.jl#870. Please wait for this PR to be merged before merging this one.

Summary

This updates AtomixMetalExt so atomic operations on Metal arrays preserve the memory ordering requested through Atomix/UnsafeAtomics.

The extension previously dispatched to Metal atomic operations without forwarding the requested ordering. That makes relaxed counter-style atomics work, but it prevents acquire/release synchronization patterns from being represented correctly on Metal.

Changes:

• Map Atomix/UnsafeAtomics orderings to Metal memory orders:
  • unordered / monotonic -> relaxed
  • acquire -> acquire
  • release -> release
  • acq_rel / acquire_release -> acq_rel
  • seq_cst / sequentially_consistent -> seq_cst
• Pass the mapped ordering through modify!.
• Pass success and failure orderings through replace!.
• Implement Metal-backed get and set! using ordered Metal load/store operations.
• Preserve relaxed Metal atomics for unordered/monotonic requests.
• Add Metal extension tests covering get/set and ordering propagation.

Why

Atomix exposes memory ordering as part of its atomic API, and downstream GPU code relies on those orderings for synchronization-sensitive algorithms. In particular, algorithms that publish ordinary device-memory writes through an atomic operation need acquire/release or stronger semantics to make the writes visible to another workitem. This was my case when building a TLAS on GPU for performing all hits computations with a ray tracing algorithm (using RayCore).

Forwarding the requested ordering to Metal fixes those synchronization cases while keeping unordered/monotonic atomics mapped to relaxed Metal operations for fast counters and reductions.

Validation

• Atomix Metal extension tests passed against a patched local Metal checkout (available here: Add ordered atomics JuliaGPU/Metal.jl#870).
• Downstream RayCore.jl TLAS/BLAS Metal repro passed with Atomix orderings wired through to Metal.
• ArchimedLight (my package) derived static TLAS comparison passed: CPU-built/adapted and direct Metal-built TLAS match for nodes, instances, all BLAS buffers, descriptors, and root AABB.