A tuplet bracket must not span a barline

A tuplet is always contained within a single bar — it never brackets across a barline. If a rhythmic figure spanning what would be a tuplet's duration crosses a bar boundary, it must be split into two separate tuplet groups tied together.

A tuplet's `indices` array includes notes on both sides of a derived barline position — e.g. `indices: [7, 8, 9]` where the true barline (by meter arithmetic) falls between indices 7 and 8.

Gould, Behind Bars, ch. 'Tuplets', tuplet brackets do not cross barlines; Ross, The Art of Music Engraving, agrees as standard practice with no noted exception.

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