A beam group must never contain notes from two different voices, even when they share a stave and align rhythmically

A beam is a single voice's rhythmic grouping. Two voices that happen to move in identical rhythm on a shared stave (common in homorhythmic chorale writing) are still beamed SEPARATELY — each voice gets its own beam object, even if the two beams end up visually parallel and close together.

A beam object constructed from noteObjects drawn from two different voices' arrays, which would force both voices' notes onto one stem-side and destroy the two-voice reading entirely.

Universal engraving convention (Gould, Read, Ross); also structurally how VexFlow's `Beam.generateBeams` works — it operates on one voice's note array at a time and has no cross-voice grouping mode.

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