A fermata in an ensemble score must be aligned across every simultaneously-active stave

When one instrument holds a fermata, every other instrument sounding at that same beat holds one too, so the ensemble stops and resumes together; an un-mirrored fermata on only one stave is a coordination error.

In a multi-instrument `Score`/`Measure` (Model C, e.g. an orchestration-lesson seed work), instrument "Violin I" has `ornament`/articulation `fermata` at beat 3 of measure 40, but "Cello" (also active, not resting, at that beat) has no fermata at the corresponding beat — on repeat listening the cello would plow through while the violin holds.

Gould, Behind Bars, ch. Fermatas — fermatas in full scores are aligned across all staves at that point (barring an explicitly solo cadenza passage, which is its own distinct notational device); Ross, The Art of Music Engraving, gives the same alignment rule for full-score fermatas.

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