Compound meters must beam in groups of three (or the declared subdivision count), reflecting the dotted-note beat, not the raw eighth-note count

In 6/8, 9/8, 12/8 (and other compound meters), the beat is a dotted note and eighth notes beam in groups of three per beat — never in groups of two, which would visually read as simple meter.

Confirmed by the code's own history comment (lines 1191-1194): '31 blocks across 15 files were being beamed as 4/4 while carrying 6/8, 2/4, 2/2, 3/4, 5/4 or 5/8' before `getDefaultBeamGroups(timeSignature)` was wired in — this exact fault already occurred in production content and was fixed generically, but nothing currently regression-tests that every compound-meter block still gets 3-groupings rather than silently regressing to 2-groupings on a future refactor.

Gould, Behind Bars, ch. 'Beaming' and 'Time signatures'.

This rule needs the notated content and the rendered result together — the score says what should happen, the page says whether it did.