'C' (common time) and 'C|' (cut time / alla breve) must resolve to the correct beat unit for every downstream computation, not just the correct total bar length

Common time (C) is 4/4 with the quarter note as the beat; cut time (C|, alla breve) is 2/2 with the HALF note as the beat — same total bar duration in whole-note terms as 4/4, but the beat unit, and therefore beaming, primary-beat-crossing rules, and metronome semantics, are all doubled relative to plain 4/4.

Gould, Behind Bars, ch. 'Time signatures', common time and cut time; universally agreed across Read, Ross, and Stone.

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