Baseline spelling rule: prefer the letter name diatonic to the current key

For a pitch class with two plausible spellings, prefer whichever is diatonic to the current key; if neither/both are diatonic, prefer the key's sharp/flat accidental-preference side.

A note in F major spelled D#4 instead of the contextually-relevant Eb4 (e.g. a borrowed iv from F minor) -- the existing preferredSpelling() would already catch this specific diatonic-membership case correctly today.

Gould, general spelling-by-key-context guidance.

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