Half-note meters (2/2, 3/2, 4/2) group by the half-note beat

In alla breve / cut time, the beat unit is the half note, so beams group eighths and sixteenths in fours/eights per half-note beat, not per quarter note. Quarter notes themselves are unbeamed in this meter (they take stems only).

2/2 content is beamed in quarter-note-sized groups (as if it were 4/4) rather than half-note-sized groups, making the alla breve feel invisible.

Gould, Behind Bars, ch. Beaming — cut-time grouping by the half-note pulse; Read, Music Notation — same. Matches VexFlow's own defaults table entries `'2/2':['1/2']`, `'3/2':['1/2']`, `'4/2':['1/2']`.

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