A system break may only fall at a barline, never mid-bar

This is close to inviolable in conventional notation: every system break coincides with the end of a complete measure in every voice simultaneously. A break mid-measure is one of the clearest 'this wasn't professionally engraved' signals.

Gould, Behind Bars (system breaks always align to barlines); Read, Music Notation; Ross, The Art of Music Engraving — universal agreement, no disagreement among authorities on this point.

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