One dynamic level per performer at a given onset

Two voices sharing one staff and one performer (soprano+alto on the treble stave of a chorale reduction, or two hands of one pianist) receive one dynamic at a given beat, not two contradicting ones, unless the passage is explicitly marked divisi/con espressione for independent parts.

In a 2-voice SATB reduction on one treble stave, voice 1 (soprano) note at beat 2 of measure 5 carries `dynamic: "f"` while voice 2 (alto), sharing the same stave and performer group, carries `dynamic: "p"` at the same onset with no divisi marking anywhere in the piece.

Gould, Behind Bars, ch. Dynamics — dynamics belong to the performer/part, not the notehead.

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