Gradus's notation model has no way to author a mid-piece clef or key-signature change, which also means it has no courtesy-clef/key-at-system-end mechanism

When a piece changes clef or key partway through, courtesy convention prints the NEW clef/key at the END of the preceding system in addition to the full clef/key at the start of the following system — a system-layout-adjacent width cost that only exists because clef/key changes exist at all.

Gould, Behind Bars (courtesy clef/key-signature conventions at system ends); Read, Music Notation (same).

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