Thin FFI wrapper around a vendored subset of ONNX Runtime's MLAS
single-precision GEMM (MlasGemmBatch).
The vendored MLAS is compiled in its standalone BUILD_MLAS_NO_ONNXRUNTIME
mode, whose threading primitives normally serialize. This crate installs a
Rayon-backed parallel-for backend (see [ensure_threading] and
vendor/shim.cpp) so MLAS keeps its own cache-aware GEMM tile partitioning
while executing the tiles across the current Rayon pool — the same pool the
rest of onnx-runtime-ep-cpu uses, so there is no oversubscription. See
docs/MLAS_SYS_SPIKE.md for the original single-thread feasibility spike.