Barline positions must be derivable from (meter, note durations, anacrusis) and checked against the hand-authored indices, not trusted blind

A barline falls exactly where the cumulative duration of notes since the last barline (offset by any anacrusis) equals one full bar's worth of beats for the meter in effect. This is arithmetic, not authorial choice — Gould, Read, and Ross all treat barline placement as fully determined by meter and rhythm (the sole discretionary exceptions, mid-bar meter changes and cadenza/senza-misura passages, are out of scope here). Gradus's OWN Public Notation API already does this correctly via `distributeIntoMeasures` in jsonToScore.ts.

Gould, Behind Bars, ch. 'Bar lines and repeat marks'; Read, Music Notation, rhythmic-notation and bar-line chapters.

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