nth_element compared through pointers into gigabytes of partials, a cache miss per comparison on one
thread (0.3 s on a 16M sweep). The ratio is now computed beside each pointer on all threads; the
comparisons and so the selection and its order are the same.
Co-Authored-By: Claude Opus 5.5 (1M context) <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01D1G8gJVAy6gp1K5Dz3NE5C