Arpeggio sequences + auto-start on first note

- Sequence mode: bars (2/4/6/8) with one chord per bar, chosen by
  functional rules (I->IV/V/vi...), diatonic cycle, or random (no immediate
  repeats); deterministic via melody seed. Chord symbols per bar in the
  score model (chordSymbols) rendered above the staff via setSection.
- Rhythm: quarters or eighths (cycling up/down through the pattern).
- Single-chord mode unchanged (whole path, overflow bars if needed).
- Auto-start: 'Start on first note (no count-in)' — step pace begins the
  run as soon as the mic detects the first note; no start press needed.
- 'New' button re-rolls the progression (new seed).
- Tests 174/174.

src/score/model.ts

@@ -91,6 +91,9 @@ export interface Score {
     readonly measures: ScoreMeasure[];
     /** Exactly one voice in v1. */
     readonly voices: ReadonlyArray<{ readonly id: 'melody'; readonly events: ScoreEvent[] }>;
+    /** Optional chord symbol per bar index (0-based), e.g. "C", "Am", "G7".
+     *  Rendered above the staff; absent = no symbol for that bar. */
+    readonly chordSymbols?: ReadonlyArray<string | null>;
     readonly source?: ScoreSource;
 }