Stem direction is dictated by voice identity, not pitch, whenever two voices share a stave

When two independent voices are notated on one staff (SATB short score, two-voice counterpoint on one stave), stem direction is assigned by voice — upper voice always up, lower voice always down — regardless of relative pitch at any instant, including voice crossings.

A shared-stave passage where stem direction flips to follow whichever voice is higher at a given beat, rather than staying fixed per voice through a crossing.

Gould, Behind Bars, Voice-pairing on a single stave (~); Ross, The Art of Music Engraving, multi-voice stem section.

This rule can be decided from pitches, durations and voices — no layout required. A validator can check it before anything is drawn.