nx_sha256_perf_test.nx

nx_sha256_perf_test.nx -- bench substrate SHA-256 on a fixed input. Hashes a deterministic 64 KiB buffer N=20 times in a loop, captures in-program wall-clock via sys_clock_gettime_mono (substrate's mono clock). Prints elapsed nanoseconds + N + bytes-per-iteration so a shell wrapper can compute MB/s. Same input + N=20 will run against system sha256sum for honest substrate-vs-incumbent comparison. expect_exit: 0 license_tier: ORIGINAL