Hand-authored barline positions and the automatic beat-based system-break calculation are two independent sources of truth that can silently disagree

There should be exactly one authority for 'where does bar N end' in a rendering pipeline — deriving barline positions from the time signature and note durations, and deriving system breaks from the SAME derived barline positions, guarantees they can never disagree.

Software-architecture observation grounded in the specific code read, not an external engraving citation — though the outcome it protects (breaks always at real barlines) is the universally agreed convention cited under break-must-fall-on-barline.

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