Add unique ambient melodies for all 19 themes

Each theme now has a distinctive looping melodic phrase (normal + wild variants)
played by the Web Audio ambient engine. This replaces the generic random-pulse
behavior that made all themes sound the same.

Go changes:
- Add melodyNote struct with freq/dur/gain fields to ambientPreset
- Add n() helper for concise melody construction
- Rewrite all 19 theme presets with unique melodic phrases:
  neon=glassy arpeggio, terminal=sparse CRT beeps, synthwave=warm minor,
  plasma=tritone shimmer, brutalist=concrete rumble, volcano=phrygian,
  aurora=high shimmer, matrix=digital ostinato, ocean=wave flow,
  dos=8-bit arpeggio, retro=VHS warmth, cosmos=vast fifths,
  retrofuture=lounge minor-7, spaceage=pure ascending, tropicale=pentatonic,
  noir=smoky minor-7, cathedral=organ thirds, surveillance=fourth alert,
  biomech=diminished unease

JS changes:
- Add melodyTimer/melodyIndex state to ambient engine
- Add playMelodyNote() with per-note gain and filter support
- Add scheduleMelodyNote() with phrase-rest gaps
- schedulePulse() now routes to melody player when melody is present
- Add sawtooth support to snonuxWaveType()
- Reset melody index on startEngine() and stopAll()

Test changes:
- TestThemeSoundPresetsAmbientPopulated now accepts melody as valid content
- TestThemeSoundPresetsAmbientValuesBounded validates melody note freq/dur

1 files changed, 12 insertions, 4 deletions

diff --git a/internal/generator/generator_test.go b/internal/generator/generator_test.go
index 3ceb72c..49d1a2f 100644
--- a/internal/generator/generator_test.go
+++ b/internal/generator/generator_test.go
@@ -158,11 +158,11 @@ func TestThemeSoundPresetsAmbientPopulated(t *testing.T) {
 		normal := preset.Ambient.Normal
 		wild := preset.Ambient.Wild
 
-		if len(normal.DroneFreqs) == 0 && len(normal.PulseFreqs) == 0 {
-			t.Errorf("theme %q ambient.Normal has no drone or pulse frequencies", name)
+		if len(normal.DroneFreqs) == 0 && len(normal.PulseFreqs) == 0 && len(normal.Melody) == 0 {
+			t.Errorf("theme %q ambient.Normal has no drone, pulse, or melody frequencies", name)
 		}
-		if len(wild.DroneFreqs) == 0 && len(wild.PulseFreqs) == 0 {
-			t.Errorf("theme %q ambient.Wild has no drone or pulse frequencies", name)
+		if len(wild.DroneFreqs) == 0 && len(wild.PulseFreqs) == 0 && len(wild.Melody) == 0 {
+			t.Errorf("theme %q ambient.Wild has no drone, pulse, or melody frequencies", name)
 		}
 	}
 }
@@ -215,6 +215,14 @@ func TestThemeSoundPresetsAmbientValuesBounded(t *testing.T) {
 					t.Errorf("theme %q ambient.%s pulseFreqs[%d]=%f; want positive", name, mode, i, f)
 				}
 			}
+			for i, m := range a.Melody {
+				if m.Freq <= 0 {
+					t.Errorf("theme %q ambient.%s melody[%d].freq=%f; want positive", name, mode, i, m.Freq)
+				}
+				if m.Dur <= 0 {
+					t.Errorf("theme %q ambient.%s melody[%d].dur=%f; want positive", name, mode, i, m.Dur)
+				}
+			}
 			if a.CutoffMin < 0 || a.CutoffMin > 10000 {
 				t.Errorf("theme %q ambient.%s cutoffMin=%f; want [0, 10000]", name, mode, a.CutoffMin)
 			}