A multi-note grace-note run renders as separate flagged notes instead of one beamed group

A run of two or more grace notes attached to a single main note (an appoggiatura pair, a slide, a turn-like figure) is conventionally beamed together as one small figure, not drawn as separate individually-flagged grace notes.

Gould, Behind Bars, ch. Grace Notes/Ornaments — multiple grace notes preceding one main note are beamed as a unit; matches VexFlow's own `GraceNoteGroup.beamNotes` API, which exists specifically to produce this beamed rendering (`gracenotegroup.ts` lines 136-148: `if (grace_notes.length > 1) { const beam = new Beam(grace_notes);... }`).

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