Giuliani studies as own material category; beam-before-draw fix

- NEW material 'giuliani': figures require a real voicing — the generator
  assigns cycle positions (bass root + upper chord tones) to DISTINCT
  strings with valid frets per bar; bars that cannot be voiced are skipped
  (deterministic), clear error when nothing fits. Patterns: p-i-m, p-i-m-a,
  p-a-m-i, a-i-m, p-i-m-a-m-i, p-m-a-m-i-m. Honors the chord pool, string
  restrictions and the tuning; one figure per beat.
- 6/8 meter added (numerator 6) across score model, settings, generators;
  meter selects now offer 4/4, 3/4, 6/8.
- Arpeggio category: Giuliani entries replaced with note-sequence
  broken-chord figures (1-3-2-1, 1-3-2-5(8), 1-5-3-5(8), 1-2-3-5-3) —
  pure pitch sequences, no voicing claims.
- Beam fix (root cause): setBeam now happens BEFORE voice.draw — VexFlow
  decides flag vs beam at draw time; registering after drawing left both
  flag and beam. Beams drawn after notes, flags suppressed correctly.

src/score/model.ts

@@ -86,7 +86,7 @@ export interface Score {
     readonly version: 1;
     readonly id: string;
     readonly title: string;
-    readonly meter: { readonly numerator: 3 | 4; readonly denominator: 4 };
+    readonly meter: { readonly numerator: 3 | 4 | 6; readonly denominator: 4 };
     readonly key: ScoreKey;
     readonly measures: ScoreMeasure[];
     /** Exactly one voice in v1. */