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/lfo_test.go

@@ -0,0 +1,90 @@
+package synth_test
+
+import (
+	"math"
+	"testing"
+
+	"github.com/netsynth/netsynth/synth"
+)
+
+func TestLFOBounds(t *testing.T) {
+	lfo := synth.NewLFO(1.0, 0.5, 44100) // 1 Hz, depth 0.5
+	for i := 0; i < 44100; i++ {
+		val := lfo.Advance()
+		if val < -0.5 || val > 0.5 {
+			t.Errorf("LFO value %v out of bounds [-0.5, 0.5] at sample %d", val, i)
+			break
+		}
+	}
+}
+
+func TestLFOPeriod(t *testing.T) {
+	// A 1 Hz LFO should complete one cycle in exactly SampleRate samples
+	lfo := synth.NewLFO(1.0, 1.0, 44100)
+	// Advance to first zero-crossing (quarter period)
+	var firstPositive float64
+	for i := 0; i < 44100; i++ {
+		val := lfo.Advance()
+		if i == 0 {
+			firstPositive = val
+		}
+		// After one full cycle, value should be close to the first value
+		if i == 44099 {
+			lastVal := val
+			// They won't be exactly equal due to phase advancement, but should be close
+			if math.Abs(lastVal-firstPositive) > 0.01 {
+				t.Errorf("LFO not periodic: first=%v, after 1 cycle=%v", firstPositive, lastVal)
+			}
+		}
+	}
+}
+
+func TestPitchLFOFreq(t *testing.T) {
+	// 0 semitones should return baseHz unchanged
+	if synth.PitchLFOFreq(440.0, 0.0) != 440.0 {
+		t.Errorf("PitchLFOFreq(440, 0) should be 440, got %v", synth.PitchLFOFreq(440.0, 0.0))
+	}
+	// 12 semitones = one octave up
+	result := synth.PitchLFOFreq(440.0, 12.0)
+	if math.Abs(result-880.0) > 0.01 {
+		t.Errorf("PitchLFOFreq(440, 12) should be 880, got %v", result)
+	}
+	// Small detuning: 0.1 semitones
+	result = synth.PitchLFOFreq(440.0, 0.1)
+	if result <= 440.0 || result >= 445.0 {
+		t.Errorf("PitchLFOFreq(440, 0.1) should be slightly above 440, got %v", result)
+	}
+}
+
+func TestLFOConfigForGroup(t *testing.T) {
+	cfg := synth.LFOConfigForGroup("Web")
+	if cfg.PitchRate == 0 {
+		t.Error("Web group should have non-zero PitchRate")
+	}
+	if cfg.TremoloRate == 0 {
+		t.Error("Web group should have non-zero TremoloRate")
+	}
+	// Unknown group should return default
+	cfg2 := synth.LFOConfigForGroup("NonexistentGroup")
+	if cfg2.PitchRate == 0 {
+		t.Error("fallback config should have non-zero PitchRate")
+	}
+}
+
+func TestGroupLFORatesIncommensurable(t *testing.T) {
+	// Verify that no two groups share the exact same pitch or tremolo rate
+	groups := []string{"Infrastructure", "Web", "Mail", "Remote Access", "File Transfer", "Database", "VoIP", "Unknown"}
+	pitchRates := make(map[float64]string)
+	tremoloRates := make(map[float64]string)
+	for _, g := range groups {
+		cfg := synth.LFOConfigForGroup(g)
+		if prev, exists := pitchRates[cfg.PitchRate]; exists {
+			t.Errorf("groups %q and %q share PitchRate=%v", prev, g, cfg.PitchRate)
+		}
+		pitchRates[cfg.PitchRate] = g
+		if prev, exists := tremoloRates[cfg.TremoloRate]; exists {
+			t.Errorf("groups %q and %q share TremoloRate=%v", prev, g, cfg.TremoloRate)
+		}
+		tremoloRates[cfg.TremoloRate] = g
+	}
+}