VexFlow's own strict-mode over/underfull-bar detection is deliberately disabled and nothing replaces it

A professional engraving tool must reject or clearly flag a bar whose notated content doesn't sum to the declared meter's length — the most basic rhythm-notation correctness check there is.

Baseline correctness assumed by Gould, Read, Ross, and Stone alike; VexFlow itself provides `Voice.setStrict(true)` specifically to enforce this, and LilyPond/Dorico/Finale all reject or visibly warn on overfull/underfull bars by default.

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