feat(synth): add LFO modulation, ADSR envelopes, pentatonic tuning, and soft limiter

Replace static EMA-smoothed drones with an evolving ambient soundscape:
- ADSR envelope system with sustained (2s attack, 4s release) and bursty
  (30ms attack, no sustain) modes per protocol group
- LFO pitch wobble and amplitude tremolo with incommensurable rates per
  group (Eno technique) so modulation patterns never repeat
- C major pentatonic frequency tuning (just intonation) — any combination
  of active protocols sounds consonant
- tanh soft limiter on master output prevents clipping
- Sync all documentation: README, PROJECT.md, ARCHITECTURE.md, v1.2
  requirements traceability

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

synth/bank_test.go

@@ -129,24 +129,24 @@ func TestStereoPan(t *testing.T) {
 	}
 }
 
-func TestMultipleWindowsEMAConvergence(t *testing.T) {
+func TestMultipleWindowsEnvelopeConvergence(t *testing.T) {
 	b := NewBank(1.0, ClassFreqConfigs)
 	counts := make(map[classify.TrafficClass]int64)
-	counts[classify.ClassICMP] = 100
+	counts[classify.ClassHTTPS] = 100 // sustained protocol — slow attack
 	snap := classify.WindowSnapshot{
 		Counts:       counts,
 		TotalPackets: 100,
 		WindowIndex:  0,
 	}
-	// Compute RMS for first and last window render
+	// Compute RMS for first window render
 	rmsFirst := windowRMS(b.RenderWindow(snap))
-	// Render 4 more windows with the same snapshot
+	// Render more windows to let ADSR attack build up (2s attack at 500ms/window ≈ 4 windows)
 	var rmsLast float64
-	for i := 0; i < 4; i++ {
+	for i := 0; i < 8; i++ {
 		rmsLast = windowRMS(b.RenderWindow(snap))
 	}
 	if rmsLast <= rmsFirst {
-		t.Errorf("EMA should converge upward: rmsFirst=%v, rmsLast=%v", rmsFirst, rmsLast)
+		t.Errorf("envelope should converge upward: rmsFirst=%v, rmsLast=%v", rmsFirst, rmsLast)
 	}
 }
 
@@ -161,9 +161,9 @@ func TestNewBankDynamicGain(t *testing.T) {
 	if len(b.layers) != 3 {
 		t.Errorf("NewBank with 3 configs has %d layers, want 3", len(b.layers))
 	}
-	// Verify gainPerLayer is 1/3
-	expected := 1.0 / 3.0
-	if b.gainPerLayer != expected {
+	// Verify gainPerLayer accounts for tremolo headroom: 1/(3 * 1.2)
+	expected := 1.0 / (3.0 * (1.0 + maxTremoloDepth))
+	if math.Abs(b.gainPerLayer-expected) > 1e-12 {
 		t.Errorf("gainPerLayer = %v, want %v", b.gainPerLayer, expected)
 	}
 }