LKlight-GPU v1.2.1 — Apple Silicon Metal GPU backend (7.6× vs CPU grid on M4)

LKlight-GPU v1.2.1

Apple Silicon Metal GPU backend (--features metal) — same-source, portable,
bit-consistent acceleration for macOS arm64.

Added

• Metal GPU batch scorer for Apple Silicon (feature = "metal", macOS only).
  New src/metal/lk_metal.m (Objective-C host + embedded MSL kernels, C ABI
  lk_metal_available/create/score/destroy) and src/metal_score.rs (Rust
  binding mirroring gpu_score.rs), wired into the engine scoring path with a
  three-tier fallback metal → cuda → CPU grid.
  • The per-atom near-field (10 Å cell-list with clamped electrostatics,
    Lennard-Jones and clash terms) and far-field (0.5 Å potential, trilinear
    interpolation) kernels are a line-for-line MSL port of the CUDA
    full_score.cu/far_field.cu numerics used by the Windows/Linux GPU
    engines, so GPU behaviour is consistent across vendors.
  • Apple GPUs have no fast fp64, so rigid-body pose transforms stay on the CPU
    in f64 and the transformed coordinates are uploaded to a shared MTLBuffer
    (zero-copy on unified memory); only the per-atom f32 accumulation runs on
    the GPU — the same f32 quantisation budget as the CUDA backend.
  • Layout: one threadgroup per pose × 128 threads with a strided atom loop and
    tree reduction (the tuned POC configuration), 14 packed buffers total.
  • If no Metal device is available (or the build lacks the feature) the engine
    transparently falls back to the CPU grid path; the fallback result is
    bit-identical to the standalone LKlight-grid engine (verified).
  • No default-build change: plain cargo build --release output is unchanged
    from v1.2.0 (CPU-only); Metal is opt-in via --features metal.

Measured (Mac mini M4, 10-core, 32 GB, unified memory; 1AZP, dna,

1000 glowworms × 1000 steps, seed 324324 — same-slot runs)

• End-to-end 6.22 s (≈ 6.2 ms/step) vs CPU grid 47.10 s and exact
  219.29 s on the same machine → ≈ 7.6× vs CPU grid, ≈ 35.3× vs exact.
• Best energy −7254.95161080, bit-identical to the RTX 5090 CUDA engine
  final value (both run the same-source f32 batch kernel); deep-pose relative
  deviation vs CPU grid stays in the documented ~1e-4 f32 class.
• Binary depends only on system frameworks (Metal, Foundation, objc,
  libSystem) — ad-hoc linker-signed, no absolute paths or brew libraries, so
  it can be copied to any Apple Silicon Mac and run as-is.

Developer-only

• dump_metal_dataset (env-gated in the Score trait) and the poc_dump bin
  export the exact dataset (receptor / cell-list / far-field / N transformed
  poses + per-pose CPU reference scores) used to develop and validate the Metal
  kernels. Not used by the normal engine path.