Non-SATB stem direction must be requested via VexFlow's auto_stem flag, not left to an omitted stem_direction default

Where no cross-voice disambiguation is needed, stem direction is pure pitch-vs-middle-line arithmetic with no exceptions — exactly the calculation VexFlow's own `StaveNote.calculateOptimalStemDirection` already performs when explicitly requested via `auto_stem: true`.

Engineering precedent, verified directly against VexFlow source rather than assumed: `StaveNote`'s constructor (stavenote.ts:444-449) is `if (noteStruct.auto_stem) { this.autoStem.

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