nx_native_matmul_long_bench.nx

nx_native_matmul_long_bench.nx -- LONG matmul bench for Stabilizer signal. The 256x256 version (nx_native_matmul_bench.nx) ran ~0.1s and the paired Stabilizer found CI 0.41-10.73x (no defensible signal -- runtime too short relative to WSL+gcc-O0 layout noise floor of 79%). This version scales to 512x512 (= 8x ops, 134M mul-adds). Target runtime: ~0.5s gcc-O0, ~1.5-3s Nishi -- enough multiples of timing granularity to escape coarse-grained noise. Per [[reference-berger-performance-matters-talk-archived]]: runtime must be much larger than measurement granularity for the noise floor as a fraction of mean to shrink to defensible levels.