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/layer.go

@@ -20,7 +20,11 @@ type Layer struct {
 }
 
 // NewLayer creates a Layer for the given config using the specified sample rate and EMA time constant (tau in seconds).
+// If cfg.WaveformType is not WaveformCustom, harmonics are resolved from the preset at construction time.
 func NewLayer(cfg FreqConfig, sampleRate int, tau float64) *Layer {
+	if cfg.WaveformType != WaveformCustom {
+		cfg.Harmonics = WaveformPresetHarmonics(cfg.WaveformType, cfg.BaseHz, sampleRate)
+	}
 	return &Layer{
 		Config:  cfg,
 		Osc:     NewOscillator(cfg.BaseHz, sampleRate),