D.C./D.S./coda/segno flags must be internally consistent and point at a real target

Da Capo (return to the beginning) and Dal Segno (return to the segno sign) are mutually exclusive instructions at a single point; "To Coda" requires an actual coda sign to jump to; Dal Segno requires an actual segno sign earlier in the piece.

A measure sets both `dalSegno: true` and `dalCapo: true` (contradictory — "go to the segno" and "go to the beginning" cannot both be the instruction at the same point), or sets `toCoda: true` while no measure anywhere in the piece has `coda: true`, or sets `dalSegno: true` while no measure has `segno: true`.

Gould, Behind Bars, ch. Repeat Marks (D.C./D.S./Coda/Segno) — states these as distinct, single-purpose directions that must resolve to a real target.

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