Accidentals track independently per voice on a shared stave

When multiple voices share one stave (SATB, or Gradus's voice2/voice3/voice4), an accidental in one voice does not cancel or apply to the same letter/octave in another voice within the same bar.

Gould, 'Divisi and multiple voices on one stave'; Read and Ross concur -- standard four-part chorale practice, directly relevant to Gradus's chorale-harmony curriculum.

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