Grace-note accidentals follow the same bar/key/voice-context rules as full-size notes

A grace-note accidental is scoped and displayed by the same bar-local, key-aware, voice-scoped rules as any other note -- it is not exempt from the accidental-scope rules above.

Gould, 'Ornaments' chapter, explicit diagrams; Read ch.9, same convention.

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