A voice's notehead must not fall inside a neighboring voice's stem corridor

A stem occupies a thin vertical corridor (stemThickness, SMuFL default ~0.12sp) from notehead to beam/flag; another voice's notehead landing inside that corridor at the same x reads as a smear even if the noteheads themselves don't touch.

Voice 2's whole note sits directly under voice 1's stem at the same x-column, with the stem's line passing through the whole-notehead's bbox.

General engraving practice; SMuFL engravingDefaults.stemThickness as the corridor width; Ross, The Art of Music Engraving on part-combining collisions.

This rule is about where marks land on the page, so it can only be judged after engraving. Collision and spacing faults live here.