test_topp proves correctness; bench_topp measures the latency claim. It re-implements
the OLD dist_build (full-vocab qsort) inline on a private buffer and times it against
the REAL new dist_build over identical inputs in one process: V=151936, temp=0.7,
3 shapes (realistic / uniform / plateau) x 4 nuc values (0.5/0.9/0.95/0.99), 2000
timed reps each, median reported. Deliberately NOT in TEST_BINS -- it's a microbench,
not a gate. Build on demand: make tests/bench_topp && ./tests/bench_topp
woolcoxm