Volta (first/second ending) brackets must be well-formed and sequentially numbered

A volta bracket that starts must close before the next repeat-end (or before the piece ends); within one repeat group, endings are numbered sequentially starting at 1 ("1.", "2.", …), not skipping or repeating a number.

A measure sets `voltaStart: 1` but no later measure ever sets `voltaEnd: true` before the piece ends (an orphaned, never-closed bracket) — or a repeat group's two endings are numbered `voltaStart: 1` then `voltaStart: 3` (skipping 2) instead of 1 then 2.

Gould, Behind Bars, ch. Repeat Barlines / First and Second Endings — both the closure and sequential-numbering rules are stated explicitly, matching universal publishing practice.

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