Clef (and key signature) repeat at the start of every system, regardless of whether either changed

Unlike a clef CHANGE (which is a genuine mid-passage event), the PREVAILING clef and key signature are conventionally re-stated at the start of every system in a multi-system piece, whether or not anything changed since the previous system — a reader beginning a new line should never have to scroll/look back to know the current clef or key.

A system that omits its clef or key signature because 'nothing changed since the last system' — this would be a genuine error, but Gradus's current renderer does not make it.

Gould, Behind Bars, and Ross, The Art of Music Engraving, both state this as universal modern practice. This is uncontroversial and is exactly what Gradus's own renderer already does correctly.

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