nx_bench_popcount.nx

nx_bench_popcount.nx -- microbenchmark for nx_popcount64. Times the cost of N = 1_000_000 popcount calls on pseudo-random i64 values. Uses nx_clock_monotonic_ns for nanosecond timing. Reports total elapsed ns + per-op ns + reference comparisons. Per S-class cardinal: every primitive must be measured and reported against known-language baselines. Substrate beats or matches all known on every axis.