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/envelope_test.go

@@ -0,0 +1,103 @@
+package synth_test
+
+import (
+	"testing"
+
+	"github.com/netsynth/netsynth/synth"
+)
+
+func TestEnvelopeStartsIdle(t *testing.T) {
+	env := synth.NewEnvelope(synth.SustainedEnvParams, synth.SampleRate, 1.0)
+	if env.State() != synth.EnvIdle {
+		t.Errorf("expected EnvIdle, got %v", env.State())
+	}
+	if env.Level() != 0 {
+		t.Errorf("expected level=0 in idle, got %v", env.Level())
+	}
+}
+
+func TestEnvelopeTriggerStartsAttack(t *testing.T) {
+	env := synth.NewEnvelope(synth.SustainedEnvParams, synth.SampleRate, 1.0)
+	env.Trigger()
+	if env.State() != synth.EnvAttack {
+		t.Errorf("expected EnvAttack after Trigger, got %v", env.State())
+	}
+}
+
+func TestEnvelopeAttackReachesPeak(t *testing.T) {
+	env := synth.NewEnvelope(synth.SustainedEnvParams, synth.SampleRate, 1.0)
+	env.Trigger()
+	// Advance through the full attack phase (2 seconds)
+	for i := 0; i < synth.SampleRate*3; i++ {
+		env.Advance()
+	}
+	if env.Level() < 0.8 {
+		t.Errorf("expected level >= 0.8 after attack, got %v", env.Level())
+	}
+}
+
+func TestEnvelopeReleaseDecays(t *testing.T) {
+	env := synth.NewEnvelope(synth.SustainedEnvParams, synth.SampleRate, 1.0)
+	env.Trigger()
+	// Build up
+	for i := 0; i < synth.SampleRate*3; i++ {
+		env.Advance()
+	}
+	peakLevel := env.Level()
+
+	env.Release()
+	// Advance through release (4 seconds)
+	for i := 0; i < synth.SampleRate*6; i++ {
+		env.Advance()
+	}
+	if env.Level() >= peakLevel*0.3 {
+		t.Errorf("expected level to decay well below peak after release, peak=%v, current=%v", peakLevel, env.Level())
+	}
+}
+
+func TestBurstyEnvelopeNoSustain(t *testing.T) {
+	env := synth.NewEnvelope(synth.BurstyEnvParams, synth.SampleRate, 1.0)
+	env.Trigger()
+	// Advance 2 seconds — bursty should have attacked, decayed (sustain=0), and be releasing
+	for i := 0; i < synth.SampleRate*2; i++ {
+		env.Advance()
+	}
+	// Should be very quiet (in release or idle)
+	if env.Level() > 0.1 {
+		t.Errorf("bursty envelope should be near zero after 2s, got %v (state=%v)", env.Level(), env.State())
+	}
+}
+
+func TestEnvelopeIdleOutputsZero(t *testing.T) {
+	env := synth.NewEnvelope(synth.SustainedEnvParams, synth.SampleRate, 1.0)
+	for i := 0; i < 100; i++ {
+		val := env.Advance()
+		if val != 0 {
+			t.Errorf("idle envelope should output 0, got %v at sample %d", val, i)
+			break
+		}
+	}
+}
+
+func TestEnvelopeRetriggerFromRelease(t *testing.T) {
+	env := synth.NewEnvelope(synth.SustainedEnvParams, synth.SampleRate, 1.0)
+	env.Trigger()
+	for i := 0; i < synth.SampleRate*3; i++ {
+		env.Advance()
+	}
+	env.Release()
+	for i := 0; i < synth.SampleRate; i++ {
+		env.Advance()
+	}
+	levelBeforeRetrigger := env.Level()
+
+	// Re-trigger
+	env.Trigger()
+	for i := 0; i < synth.SampleRate*3; i++ {
+		env.Advance()
+	}
+	if env.Level() <= levelBeforeRetrigger {
+		t.Errorf("re-triggered envelope should rise above release level, before=%v, after=%v",
+			levelBeforeRetrigger, env.Level())
+	}
+}