Beams break at rests (Gradus's pedagogical default)

A beam group does not extend across a rest — the rest breaks the beam, each side gets its own (shorter) beam or reverts to flags. Traditional default; some contemporary instrumental engraving (Stone) permits beaming through short rests, but that is a stylistic departure not appropriate for Gradus's pedagogical content.

A beam spanning across a rest tickable — would only occur if a future code change passes `beam_rests: true` to `Beam.generateBeams`.

Gould, Behind Bars, Beams chapter (traditional default: break at rests); Stone, Music Notation in the Twentieth Century (documents the contemporary alternative as a departure). Recommend Gould's default for Gradus — matches current renderer behavior.

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