The fingering field is declared but never rendered anywhere

Fingering numerals are a standard attachment placed directly above or below the notehead (above the staff for the right hand/treble convention, below for the left hand/bass, or immediately adjacent for string instruments), used throughout instructional keyboard and string literature — exactly the audience Gradus serves.

Gould, Behind Bars, ch. Fingering — placement and sizing conventions for keyboard/string fingering numerals.

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