fix(09-01): add TestGroupFieldPopulated, fix oscillator normalization, update tests

- Add TestGroupFieldPopulated test to verify all ClassFreqConfigs have non-empty Group (GRP-01)
- Change TestHarmonicsNonEmpty threshold from < 2 to < 1 (accepts single-harmonic sine entries)
- Fix oscillator Advance() to use math.Abs(h.Amplitude) for normalization weight accumulation:
  triangle wave uses alternating-sign amplitudes; signed sum underestimates totalWeight causing
  output to exceed [-1,1] bounds when using WaveformPresetHarmonics (Rule 1 bug fix)
- Update TestStereoPan to use ClassSSH (pan=-0.7) instead of ClassDHCP (pan changed to 0.1)
- Update TestNewBankCustomConfigNoClip: passes after oscillator normalization fix
- Fix TestLoadPartialOverrideFrequency: derive expected WaveformType from defaults (not hardcoded 0)
- Fix TestAutoFreqSkipsBuiltins: derive expected HTTPS BaseHz from defaults (150.0 in Phase 9)

synth/oscillator.go

@@ -15,12 +15,15 @@ func NewOscillator(freq float64, sampleRate int) *Oscillator {
 }
 
 // Advance returns one sample: fundamental + harmonics summed and normalized to [-1, 1].
+// Normalization uses sum of absolute amplitudes so that alternating-sign harmonic series
+// (e.g. triangle wave) are correctly bounded. Without math.Abs, signed cancellation
+// produces an inflated normalization denominator that causes output to exceed [-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
+		totalWeight += math.Abs(h.Amplitude)
 	}
 	o.phase += o.freq / o.sr
 	if o.phase >= 1.0 {