Add v2 Game Boy sound engine (isolated from v1)

- Authentic DMG-CPU sound chip implementation:
  - 4 Pulse channels with duty cycle control (12.5%, 25%, 50%, 75%)
  - 2 Wave channels with 4-bit wavetables
  - 2 Noise channels with LFSR (7-bit and 15-bit modes)

- GameBoy Colorizer effect chain:
  - Low-pass filter (natural GB rolloff)
  - Bit-crushing (4-bit DAC simulation)
  - Sample rate reduction
  - Saturation and high-pass filter
  - Presets: DMG, GBC, GBA, Clean

- Intelligent MIDI processing:
  - Track analysis and role detection (bass, lead, drums, etc.)
  - Automatic channel mapping to GB channels
  - Chord arpeggiator for polyphony handling
  - GameBoy Arranger for fuller sound

- BitMidi search integration
- Completely isolated from v1 (no changes to src/)

src-v2/audio/synthesis/DutyCycle.ts

@@ -0,0 +1,82 @@
+/**
+ * Game Boy Duty Cycle Implementation
+ * 
+ * The GB pulse channels support 4 duty cycle patterns.
+ * These exact duty ratios give the Game Boy its distinctive sound.
+ */
+
+/**
+ * Duty cycle ratios for the 4 GB patterns
+ * 12.5% - Very thin, buzzy, laser-like sound
+ * 25%   - Classic chiptune sound, bright and punchy
+ * 50%   - Full square wave
+ * 75%   - Same as 25% but inverted
+ */
+export const DUTY_RATIOS = [0.125, 0.25, 0.5, 0.75] as const;
+
+export type DutyIndex = 0 | 1 | 2 | 3;
+
+/**
+ * Creates a PeriodicWave for Web Audio from a duty cycle.
+ * 
+ * Uses proper Fourier series for pulse wave:
+ * imag[n] = (2 / (π * n)) * sin(π * n * duty)
+ * 
+ * This is the mathematically correct way to synthesize pulse waves.
+ */
+export function createDutyWave(
+  dutyIndex: DutyIndex,
+  audioContext: BaseAudioContext
+): PeriodicWave {
+  const dutyRatio = DUTY_RATIOS[dutyIndex];
+  
+  // More harmonics = sharper edges (but more CPU)
+  const numHarmonics = 64;
+  
+  const real = new Float32Array(numHarmonics);
+  const imag = new Float32Array(numHarmonics);
+  
+  // DC offset = 0 for centered waveform
+  real[0] = 0;
+  imag[0] = 0;
+  
+  // Fourier series for pulse wave
+  // https://en.wikipedia.org/wiki/Pulse_wave
+  for (let n = 1; n < numHarmonics; n++) {
+    // Pulse wave Fourier coefficient
+    const coefficient = (2 / (Math.PI * n)) * Math.sin(Math.PI * n * dutyRatio);
+    imag[n] = coefficient;
+    real[n] = 0;
+  }
+  
+  return audioContext.createPeriodicWave(real, imag, {
+    disableNormalization: false
+  });
+}
+
+/**
+ * Pre-creates all 4 duty cycle waveforms for efficient reuse.
+ */
+export function createAllDutyWaves(
+  audioContext: BaseAudioContext
+): PeriodicWave[] {
+  return [
+    createDutyWave(0, audioContext),
+    createDutyWave(1, audioContext),
+    createDutyWave(2, audioContext),
+    createDutyWave(3, audioContext),
+  ];
+}
+
+/**
+ * Returns a human-readable description of each duty cycle.
+ */
+export function getDutyDescription(dutyIndex: DutyIndex): string {
+  const descriptions = [
+    '12.5% - Thin, buzzy',
+    '25% - Classic chiptune',
+    '50% - Full square',
+    '75% - Bright, punchy',
+  ];
+  return descriptions[dutyIndex];
+}