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/config_test.go

@@ -39,8 +39,8 @@ func TestFrequenciesUnique(t *testing.T) {
 
 func TestHarmonicsNonEmpty(t *testing.T) {
 	for class, cfg := range synth.ClassFreqConfigs {
-		if len(cfg.Harmonics) < 2 {
-			t.Errorf("class %q has fewer than 2 harmonics (got %d)", class, len(cfg.Harmonics))
+		if len(cfg.Harmonics) < 1 {
+			t.Errorf("class %q has no harmonics (got %d)", class, len(cfg.Harmonics))
 		}
 	}
 }
@@ -59,3 +59,11 @@ func TestClassFreqConfigsMatchAllClasses(t *testing.T) {
 			len(synth.ClassFreqConfigs), len(classify.AllClasses()))
 	}
 }
+
+func TestGroupFieldPopulated(t *testing.T) {
+	for class, cfg := range synth.ClassFreqConfigs {
+		if cfg.Group == "" {
+			t.Errorf("class %q has empty Group field in ClassFreqConfigs", class)
+		}
+	}
+}