Simple-meter beat grouping (2/4, 3/4, 4/4, and their multiples)

In simple time, beams group notes to make each metric beat visible: one beam per quarter-note beat in 4/4 and 3/4 (two eighths, or four sixteenths, per group), and correspondingly per beat in 2/4. The beam boundary must fall exactly on each beat, never spanning two beats unless a higher convention (tuplet, explicit compound feel) overrides it.

An eighth-note beam group spans notes 2 and 3 of a 4/4 bar (crossing from beat 2 into beat 3) instead of grouping 1-2 and 3-4 separately.

Gould, Behind Bars, ch. Beaming (beam groups reflect the beat); Read, Music Notation, beaming chapter — beam to the beat as the default rule of thumb; matches VexFlow's own defaults table (`'2/4':['1/4']`, `'3/4':['1/4']`, `'4/4':['1/4']`, confirmed in `beam.ts` lines 109-112).

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