Identical simultaneous rests across two voices sharing a stave must render as ONE shared rest, not two displaced ones

When two voices sharing a staff have a rest of the same duration starting at the same beat, engraving convention draws a single rest (centered, default position) shared by both voices rather than two separately-displaced rest glyphs.

Gradus currently displaces EVERY rest by voice role whenever that voice has a forced stem, with no check for whether the other voice sharing the stave has an identical simultaneous rest. A whole bar of silence shared by both S and A on a chorale would draw as two offset rest glyphs instead of one shared rest.

Gould, Behind Bars: 'Separately stemmed parts share rests wherever possible.' Explicit, not a general inference.

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