Additive/irregular meters (5/8, 7/8, etc.) need an explicit declared beat grouping that Gradus cannot currently author

Meters like 5/8 (2+3 or 3+2) and 7/8 (2+2+3, 3+2+2, or 2+3+2) have no single 'natural' grouping derivable from the numerator and denominator alone — the composer must state which grouping is intended, and the engraving must follow it consistently (or explicitly re-state it if the grouping changes bar to bar).

A 7/8 lesson block relies entirely on `Beam.getDefaultBeamGroups('7/8')` (confirmed line 1202) with no author-facing way to specify 2+2+3 vs. 3+2+2 vs. 2+3+2 — whatever VexFlow's library default assumes is silently what renders.

Stone, Music Notation in the Twentieth Century, ch. on irregular/additive meters — the definitive treatment, including the convention of a small numeral grouping annotation (e.g. '2+2+3') above the time signature; Gould, Behind Bars, agrees additive meters need explicit grouping but treats it as secondary to Stone's fuller treatment.

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