feat: chord voicing theory layer

app/app/lib/chord-voicing.test.ts

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+import { describe, it, expect } from 'vitest';
+import { getPianoNotes, getGuitarVoicing } from './chord-voicing';
+
+describe('getPianoNotes', () => {
+  it('returns note names for a major chord', () => {
+    expect(getPianoNotes('C')).toEqual(['C', 'E', 'G']);
+  });
+
+  it('returns note names for Cmaj7', () => {
+    expect(getPianoNotes('Cmaj7')).toEqual(['C', 'E', 'G', 'B']);
+  });
+
+  it('returns note names for Am', () => {
+    expect(getPianoNotes('Am')).toEqual(['A', 'C', 'E']);
+  });
+
+  it('returns [] for unparseable chord', () => {
+    expect(getPianoNotes('???')).toEqual([]);
+  });
+
+  it('returns [] for empty string', () => {
+    expect(getPianoNotes('')).toEqual([]);
+  });
+});
+
+describe('getGuitarVoicing', () => {
+  it('returns voicing for E major (open position, baseFret=0)', () => {
+    const v = getGuitarVoicing('E');
+    expect(v).not.toBeNull();
+    expect(v!.baseFret).toBe(0);
+    expect(v!.frets).toEqual([0, 2, 2, 1, 0, 0]);
+  });
+
+  it('returns voicing for Am (open position, baseFret=0)', () => {
+    const v = getGuitarVoicing('Am');
+    expect(v).not.toBeNull();
+    expect(v!.baseFret).toBe(0);
+    expect(v!.frets).toEqual([null, 0, 2, 2, 1, 0]);
+  });
+
+  it('transposes Bm correctly (A-shape, shift=2)', () => {
+    const v = getGuitarVoicing('Bm');
+    expect(v).not.toBeNull();
+    // Am shifted up 2: [null,2,4,4,3,2], baseFret=2
+    expect(v!.baseFret).toBe(2);
+    expect(v!.frets).toEqual([null, 2, 4, 4, 3, 2]);
+  });
+
+  it('transposes G major correctly (E-shape, shift=3)', () => {
+    const v = getGuitarVoicing('G');
+    expect(v).not.toBeNull();
+    // E major shifted up 3: [3,5,5,4,3,3], baseFret=3
+    expect(v!.baseFret).toBe(3);
+    expect(v!.frets).toEqual([3, 5, 5, 4, 3, 3]);
+  });
+
+  it('returns null for unknown quality', () => {
+    // 'add9' is not in the voicing map
+    expect(getGuitarVoicing('Cadd9')).toBeNull();
+  });
+
+  it('returns null for unparseable chord', () => {
+    expect(getGuitarVoicing('???')).toBeNull();
+  });
+});

app/app/lib/chord-voicing.ts

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+import { Chord, Note } from 'tonal';
+import { GUITAR_VOICINGS } from './guitar-voicings';
+
+export interface GuitarVoicing {
+  /** Absolute fret numbers per string (low→high); null = muted, 0 = open */
+  frets: (number | null)[];
+  /** Lowest fret displayed on the diagram (0 = show nut) */
+  baseFret: number;
+  /** Absolute fret to draw a barre bar across, or null */
+  barre: number | null;
+}
+
+const ROOT_STRING_CHROMA: Record<'E' | 'A', number> = {
+  E: Note.chroma('E')!,  // 4
+  A: Note.chroma('A')!,  // 9
+};
+
+/**
+ * Returns the note names (e.g. ["C","E","G"]) for a chord string.
+ * Returns [] if the chord cannot be parsed.
+ */
+export function getPianoNotes(chord: string): string[] {
+  if (!chord) return [];
+  const parsed = Chord.get(chord);
+  if (!parsed.tonic || parsed.empty) return [];
+  return parsed.notes;
+}
+
+/**
+ * Returns a transposed GuitarVoicing for a chord string, or null if the
+ * chord quality has no template or the chord cannot be parsed.
+ */
+export function getGuitarVoicing(chord: string): GuitarVoicing | null {
+  if (!chord) return null;
+  const parsed = Chord.get(chord);
+  if (!parsed.tonic || parsed.empty) return null;
+
+  const template = GUITAR_VOICINGS[parsed.type];
+  if (!template) return null;
+
+  const rootChroma = ROOT_STRING_CHROMA[template.rootString];
+  const tonicChroma = Note.chroma(parsed.tonic);
+  if (tonicChroma === undefined) return null;
+
+  const shift = (tonicChroma - rootChroma + 12) % 12;
+
+  return {
+    frets: template.frets.map((f) => (f === null ? null : f + shift)),
+    baseFret: shift,
+    barre: template.barre === null ? null : template.barre + shift,
+  };
+}