The final measure of a piece must end with a final barline, never a plain single barline or a dangling repeat sign

The very last barline of a complete piece (or a complete movement) is always a final/closing double barline (thin-thick), never a plain single barline and never a forward-only repeat sign left unresolved.

The last `Measure` in a `Score`'s measure array has `barlineEnd` left at the default `'normal'` (a plain single line) or, worse, still set to `'repeat-end'` with no subsequent D.C./D.S. instruction that would actually cause a return to that repeat — a piece that structurally just stops without a closing bar.

Gould, Behind Bars, ch. Barlines — the final barline of a piece is always the double thin-thick closing bar; Ross, The Art of Music Engraving, states the same as universal convention.

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