Hairpins, slurs, and tuplets are voice-1-only in the JSX renderer — flag any attempt to imply otherwise

In genuine polyphonic/SATB writing, a crescendo can apply to any individual voice, not only the top line.

Gould, Behind Bars — dynamics and hairpins are properties of a part/voice, not exclusively the melody line; standard chorale and orchestral engraving routinely gives the alto or bass its own independent hairpin.

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