Function select_gemm_backend 

pub fn select_gemm_backend(m: usize, k: usize, n: usize) -> GemmBackend

Select the GEMM backend for an m×k×n matmul: coopmat when armed+usable and all dims are 8-mult (the tensor-core tile — batched prefill, not the m=1 decode GEMV); else cooperative GEMV when enabled; else naive. Pure + total, so it is unit-tested without a GPU.