The ASU-key sample over every partial ran on one thread (0.2-1 s of the merge tail). Each image's
parts are now taken on the workers and joined in image order, so the sort sees the same parts in the
same order.
Co-Authored-By: Claude Opus 5.5 (1M context) <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01D1G8gJVAy6gp1K5Dz3NE5C