6/4, 9/4, 12/4 are metrically ambiguous and the naive fallback silently guesses

A numerator divisible by 3 does not by itself guarantee a compound feel. 6/4 carries two legitimate, mutually exclusive readings: (a) compound duple — 2 beats, each a dotted half note, grouped 3+3 quarters (VexFlow's actual default via its `tripleMeter` branch); or (b) simple triple with a doubled beat unit — 3 beats, each a half note, grouped 2+2+2 quarters — used when 6/4 functions as a slower relative of 3/2, including the classic hemiola against a surrounding 3/2 bar. 9/4 and 12/4 carry the analogous ambiguity (3 or 4 dotted-half beats vs. a triple/quadruple half-note reading). The composer's intended grouping must be explicit, not inferred from the numerator alone.

A hemiola passage authored in 6/4 (meant to feel as 3 groups of 2 quarters against a surrounding 3/2 bar) gets silently beamed as 2 groups of 3 quarters by `Beam.getDefaultBeamGroups`'s `tripleMeter` branch, contradicting the notated hemiola.

Gould, Behind Bars, ch. Beaming, discusses 6/4's dual reading explicitly; Stone, Music Notation in the 20th Century, discusses additive/ambiguous meters requiring explicit grouping marks. Where Gould's context-sensitive judgment and VexFlow's naive heuristic disagree, Gould's should win for audit purposes — recommend Gradus require an explicit author-declared grouping for 6/4, 9/4, 12/4 rather than trust the guess, once the override channel exists.

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