Sort keys computed once, not in the comparator: an FMA-contracted Length() can
round differently at different inlined call sites, which breaks the strict weak
ordering std::sort needs and lets its partition scan run off the array.
Co-Authored-By: Claude Opus 5 (1M context) <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_013nW6FNRP1bBJJ8pfHiByAT