Seconds inside a chord must offset noteheads left/right of the stem

When two pitches in the same chord are a 2nd apart, the higher notehead is drawn on the normal (stem-appropriate) side and the lower is displaced to the opposite side of the stem so the two notehead bounding boxes do not overlap.

A chord like [C4,D4] renders both noteheads on the same side of the stem, fully overlapping.

Gould, Behind Bars, ch. 'Note relationships' / seconds in chords; Read, Music Notation, ch. on chord notation; VexFlow's own `StaveNote.format` (called from ModifierContext.preFormat, category StaveNote — the FIRST category run, before Dot/Accidental/etc.) implements this internally.

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