Barline cancels all bar-local accidentals with no natural required

An accidental's effect ends at the next barline; the following bar's first occurrence reverts silently to key signature/natural with no cancellation natural needed.

A bar ends with G#4; the next bar's G4 carries an unnecessary explicit natural.

Gould, 'Accidentals' chapter; Read ch.6; Stone -- universal agreement across engraving manuals.

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