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.go

@@ -0,0 +1,72 @@
+package synth
+
+import "math"
+
+// LFO is a low-frequency oscillator for modulating synthesis parameters.
+// Uses sine waveform. Rate is in Hz (typically 0.01-0.5 Hz for ambient feel).
+type LFO struct {
+	phase float64
+	rate  float64 // Hz
+	depth float64 // modulation depth (interpretation depends on usage)
+	sr    float64
+}
+
+// NewLFO creates an LFO at the given rate (Hz) and depth.
+func NewLFO(rate, depth float64, sampleRate int) *LFO {
+	return &LFO{
+		rate:  rate,
+		depth: depth,
+		sr:    float64(sampleRate),
+	}
+}
+
+// Advance returns the current LFO value in [-depth, +depth] and advances the phase.
+func (l *LFO) Advance() float64 {
+	val := math.Sin(2 * math.Pi * l.phase)
+	l.phase += l.rate / l.sr
+	if l.phase >= 1.0 {
+		l.phase -= math.Floor(l.phase)
+	}
+	return val * l.depth
+}
+
+// PitchLFOFreq returns a pitch-modulated frequency given a base frequency
+// and an LFO value in semitones. For example, lfoVal=0.1 shifts pitch up by 0.1 semitones.
+func PitchLFOFreq(baseHz, lfoSemitones float64) float64 {
+	return baseHz * math.Pow(2.0, lfoSemitones/12.0)
+}
+
+// LFOConfig holds LFO parameters for a synthesis layer.
+// Each protocol gets unique, incommensurable rates so modulation patterns never repeat.
+type LFOConfig struct {
+	PitchRate     float64 // Hz, typically 0.02-0.08
+	PitchDepth    float64 // semitones, typically 0.05-0.15
+	TremoloRate   float64 // Hz, typically 0.05-0.3
+	TremoloDepth  float64 // amplitude fraction, typically 0.05-0.2
+}
+
+// layerLFOConfigs provides unique incommensurable LFO rates per protocol group.
+// Rates chosen as non-integer ratios to avoid periodic sync (Eno technique).
+var groupLFOConfigs = map[string]LFOConfig{
+	"Infrastructure": {PitchRate: 0.031, PitchDepth: 0.08, TremoloRate: 0.053, TremoloDepth: 0.12},
+	"Web":            {PitchRate: 0.043, PitchDepth: 0.10, TremoloRate: 0.071, TremoloDepth: 0.15},
+	"Mail":           {PitchRate: 0.037, PitchDepth: 0.07, TremoloRate: 0.059, TremoloDepth: 0.10},
+	"Remote Access":  {PitchRate: 0.029, PitchDepth: 0.12, TremoloRate: 0.047, TremoloDepth: 0.18},
+	"File Transfer":  {PitchRate: 0.041, PitchDepth: 0.09, TremoloRate: 0.067, TremoloDepth: 0.13},
+	"Database":       {PitchRate: 0.023, PitchDepth: 0.06, TremoloRate: 0.083, TremoloDepth: 0.10},
+	"VoIP":           {PitchRate: 0.019, PitchDepth: 0.05, TremoloRate: 0.091, TremoloDepth: 0.08},
+	"Unknown":        {PitchRate: 0.053, PitchDepth: 0.15, TremoloRate: 0.037, TremoloDepth: 0.20},
+}
+
+// DefaultLFOConfig is the fallback for groups not in the map.
+var DefaultLFOConfig = LFOConfig{
+	PitchRate: 0.033, PitchDepth: 0.10, TremoloRate: 0.057, TremoloDepth: 0.15,
+}
+
+// LFOConfigForGroup returns the LFO config for a protocol group.
+func LFOConfigForGroup(group string) LFOConfig {
+	if cfg, ok := groupLFOConfigs[group]; ok {
+		return cfg
+	}
+	return DefaultLFOConfig
+}