feat(02-01): implement synth config, oscillator, and layer

- synth/config.go: FreqConfig, HarmonicDef types; ClassFreqConfigs for all 11 traffic classes
  with frequencies in 60-800 Hz using musical intervals; pan positions per D-12
- synth/oscillator.go: phase-accumulator oscillator with additive harmonic synthesis
  and phase subtraction wrap (not math.Mod) per D-05
- synth/layer.go: EMA amplitude smoothing with tau-based alpha; whisper floor (0.03)
  prevents silence once class has been seen; UpdateTarget/AdvanceSample interface
- All 12 tests pass (TestAllClassesHaveConfig, TestFrequenciesUnique, TestEMAAmplitudeRise, etc.)

synth/oscillator.go

@@ -0,0 +1,38 @@
+package synth
+
+import "math"
+
+// Oscillator is a phase-accumulator oscillator that generates additive sine waveforms.
+type Oscillator struct {
+	phase float64
+	freq  float64
+	sr    float64
+}
+
+// NewOscillator creates an oscillator at the given frequency and sample rate.
+func NewOscillator(freq float64, sampleRate int) *Oscillator {
+	return &Oscillator{freq: freq, sr: float64(sampleRate)}
+}
+
+// Advance returns one sample: fundamental + harmonics summed and normalized to [-1, 1].
+func (o *Oscillator) Advance(harmonics []HarmonicDef) float64 {
+	sum := 0.0
+	totalWeight := 0.0
+	for _, h := range harmonics {
+		sum += h.Amplitude * math.Sin(2*math.Pi*o.phase*float64(h.Ratio))
+		totalWeight += h.Amplitude
+	}
+	o.phase += o.freq / o.sr
+	if o.phase >= 1.0 {
+		o.phase -= 1.0
+	}
+	if totalWeight > 0 {
+		return sum / totalWeight
+	}
+	return 0
+}
+
+// Phase returns the current phase (for testing).
+func (o *Oscillator) Phase() float64 {
+	return o.phase
+}