Two voices sharing one stave must not silently stack a unison/near-unison note

When two independent voices share a stave (SATB soprano+alto on the treble stave, tenor+bass on the bass stave), their noteheads must be joined into one ModifierContext so overlapping or near-overlapping pitches between the voices get offset exactly like a chord's seconds rule; treating each voice's Formatter independently silently draws both noteheads on top of each other.

Alto and soprano land on the same space with no offset because their VFVoice objects were formatted independently instead of jointly.

Gould, Behind Bars, ch. 'Voices on a single stave'.

This rule needs the notated content and the rendered result together — the score says what should happen, the page says whether it did.