feat(05-01): wire waveform resolution into NewLayer at construction time

- NewLayer resolves WaveformPresetHarmonics when cfg.WaveformType != WaveformCustom
- Preset harmonics stored in Layer.Config so AdvanceSample uses them unchanged
- WaveformCustom path preserves existing hand-tuned harmonics (backward compatible)
- TestNewLayerResolvesWaveformPreset: verifies preset fills harmonics on construction
- TestNewLayerPreservesCustomHarmonics: verifies hand-tuned harmonics are untouched
- TestSineRegressionVsCustomHarmonics: verifies WaveformSine == {Ratio:1,Amp:1.0}
- All 41 synth tests pass, encode tests unaffected

synth/waveform_test.go

@@ -127,6 +127,71 @@ func TestWaveformPresetHarmonics_SawtoothConsecutive(t *testing.T) {
 	}
 }
 
+func TestNewLayerResolvesWaveformPreset(t *testing.T) {
+	cfg := synth.FreqConfig{
+		BaseHz:       440.0,
+		WaveformType: synth.WaveformSquare,
+		// Harmonics intentionally empty — preset should be resolved
+	}
+	layer := synth.NewLayer(cfg, synth.SampleRate, 1.0)
+	if len(layer.Config.Harmonics) <= 1 {
+		t.Errorf("expected layer.Config.Harmonics to have length > 1 after preset resolution, got %d", len(layer.Config.Harmonics))
+	}
+	if layer.Config.Harmonics[0].Ratio != 1 {
+		t.Errorf("expected first harmonic Ratio=1, got %d", layer.Config.Harmonics[0].Ratio)
+	}
+}
+
+func TestNewLayerPreservesCustomHarmonics(t *testing.T) {
+	cfg := synth.FreqConfig{
+		BaseHz:    440.0,
+		Harmonics: []synth.HarmonicDef{{Ratio: 1, Amplitude: 1.0}, {Ratio: 2, Amplitude: 0.4}},
+		// WaveformType zero value = WaveformCustom
+	}
+	layer := synth.NewLayer(cfg, synth.SampleRate, 1.0)
+	if len(layer.Config.Harmonics) != 2 {
+		t.Errorf("expected exactly 2 harmonics preserved, got %d", len(layer.Config.Harmonics))
+	}
+	if layer.Config.Harmonics[1].Amplitude != 0.4 {
+		t.Errorf("expected second harmonic Amplitude=0.4, got %.4f", layer.Config.Harmonics[1].Amplitude)
+	}
+}
+
+func TestSineRegressionVsCustomHarmonics(t *testing.T) {
+	// Sine preset should produce identical output to a single-harmonic custom config
+	cfgSine := synth.FreqConfig{
+		BaseHz:       440.0,
+		WaveformType: synth.WaveformSine,
+	}
+	cfgCustom := synth.FreqConfig{
+		BaseHz:    440.0,
+		Harmonics: []synth.HarmonicDef{{Ratio: 1, Amplitude: 1.0}},
+	}
+	// Use fast-converging tau for test
+	layerSine := synth.NewLayer(cfgSine, synth.SampleRate, 0.001)
+	layerCustom := synth.NewLayer(cfgCustom, synth.SampleRate, 0.001)
+
+	// Set both to same target amplitude
+	layerSine.UpdateTarget(1, 1)
+	layerCustom.UpdateTarget(1, 1)
+
+	// Advance enough samples for EMA to converge (tau=0.001 at 44100 SR: ~44 samples to 63%)
+	for i := 0; i < 200; i++ {
+		layerSine.AdvanceSample()
+		layerCustom.AdvanceSample()
+	}
+
+	// Next 100 samples should match exactly
+	for i := 0; i < 100; i++ {
+		s1 := layerSine.AdvanceSample()
+		s2 := layerCustom.AdvanceSample()
+		if abs(s1-s2) > 1e-12 {
+			t.Errorf("sample %d: sine preset (%.10f) != custom harmonic (%.10f), diff=%.2e", i, s1, s2, abs(s1-s2))
+			break
+		}
+	}
+}
+
 // abs returns the absolute value of x.
 func abs(x float64) float64 {
 	if x < 0 {