VORTEXM4: STRMM/DTRMM at about half of Accelerate at n = 1024 #6147

STRMM and DTRMM on VORTEXM4 (SME2 kernels from #6074) are slower than the other level-3 routines also at large sizes. On an M4 Pro, one thread, n = 1024: all 16 STRMM variants run at 780-888 GFLOPS, Accelerate at 1536-1562 (0.52x); DTRMM is at 0.61x. At n = 4096 STRMM is at 0.78-0.86x. SYMM, which uses the same GEMM kernel through the level-3 driver, reaches 0.93x at n = 1024.

The difference is therefore specific to the TRMM path. Candidates, not yet measured separately:

• the TRMM kernel (gemm_kernel_sme2.c with TRMMKERNEL) zeroes ZA and stores C = alpha * ZA with a per-row scale-only store (s2_c_store_alpha), not the four-row fast path of the GEMM case;
• the triangle copies (trmm_ncopy_sme2.c, trmm_tcopy_sme2.c, ~43 GB/s, see VORTEXM4: packing dominates SME2 level-3 at small n #6146);
• driver/level3/trmm_L.c / trmm_R.c call the kernel on chunks of at most 3 x GEMM_UNROLL_N = 48 columns.

A first step is a profile that separates kernel time, copy time and driver overhead at n = 1024.