count_sliced computed the slice size from the requested thread count and
only then clamped nt to MAX_THREADS, so the last slice ended before the
file tail: raising the cap (as pick_threads now does) silently dropped
the tail from every count. Cap nt first, then derive per.
Monsters (>=256 MiB) now get one thread per core (capped at 24) instead
of a hard 16, which measures ~1.17x on the 1B-line case and is flat on
warm files. check_sliced now sweeps the cap boundary (16, 24, 25).
huntedbytheirs
af61660c42