feat(05-02): decouple NewBank from global config and fix dynamic GainPerLayer

- NewBank now accepts (tau float64, cfgs map[classify.TrafficClass]FreqConfig)
- gainPerLayer field added to OscillatorBank, computed as 1.0/float64(len(cfgs))
- RenderWindow UpdateTarget loop iterates b.layers (not classify.AllClasses())
- RenderWindow render loop uses b.gainPerLayer (not GainPerLayer constant)
- encode/mp3.go updated to pass synth.ClassFreqConfigs as default config map

encode/mp3.go

@@ -54,7 +54,7 @@ func RunSynthesis(snapshots []classify.WindowSnapshot, outputPath string) error
 	}
 
 	// Render all windows to stereo frames
-	bank := synth.NewBank(1.0) // tau=1.0s per D-07
+	bank := synth.NewBank(1.0, synth.ClassFreqConfigs) // tau=1.0s per D-07
 	var allFrames [][2]float64
 	for _, snap := range snapshots {
 		frames := bank.RenderWindow(snap)

synth/bank.go

@@ -2,22 +2,25 @@ package synth
 
 import "github.com/netsynth/netsynth/classify"
 
-// OscillatorBank holds 11 synthesis layers, one per TrafficClass.
+// OscillatorBank holds synthesis layers, one per TrafficClass in the injected config map.
 // It consumes WindowSnapshot data and renders stereo PCM frames.
 type OscillatorBank struct {
-	layers map[classify.TrafficClass]*Layer
-	tau    float64
+	layers       map[classify.TrafficClass]*Layer
+	tau          float64
+	gainPerLayer float64
 }
 
-// NewBank creates an OscillatorBank with one Layer per TrafficClass.
+// NewBank creates an OscillatorBank with one Layer per entry in cfgs.
 // tau is the EMA time constant in seconds (use 1.0 for D-07's "1-2 second" feel).
-func NewBank(tau float64) *OscillatorBank {
+// gainPerLayer is computed dynamically as 1/len(cfgs) so that all layers at max
+// amplitude sum to exactly 1.0 (no clipping), regardless of how many classes are active.
+func NewBank(tau float64, cfgs map[classify.TrafficClass]FreqConfig) *OscillatorBank {
 	b := &OscillatorBank{
-		layers: make(map[classify.TrafficClass]*Layer, NumLayers),
-		tau:    tau,
+		layers:       make(map[classify.TrafficClass]*Layer, len(cfgs)),
+		tau:          tau,
+		gainPerLayer: 1.0 / float64(len(cfgs)),
 	}
-	for _, class := range classify.AllClasses() {
-		cfg := ClassFreqConfigs[class]
+	for class, cfg := range cfgs {
 		b.layers[class] = NewLayer(cfg, SampleRate, tau)
 	}
 	return b
@@ -25,7 +28,7 @@ func NewBank(tau float64) *OscillatorBank {
 
 // RenderWindow updates amplitude targets from snap, then renders SamplesPerWindow
 // stereo frames. Each frame is [2]float64{left, right} with values in [-1, 1].
-// Per D-10: each layer gets GainPerLayer (1/11) so 11 max-amplitude layers sum to 1.0 (no clipping).
+// Each layer gets 1/N of the total gain where N is the number of layers.
 func (b *OscillatorBank) RenderWindow(snap classify.WindowSnapshot) [][2]float64 {
 	// Find max count for normalization
 	var maxCount int64
@@ -36,21 +39,20 @@ func (b *OscillatorBank) RenderWindow(snap classify.WindowSnapshot) [][2]float64
 	}
 
 	// Update target amplitudes for all layers
-	for _, class := range classify.AllClasses() {
+	for class, layer := range b.layers {
 		count := snap.Counts[class]
-		b.layers[class].UpdateTarget(count, maxCount)
+		layer.UpdateTarget(count, maxCount)
 	}
 
 	// Render frames
 	frames := make([][2]float64, SamplesPerWindow)
 	for i := range frames {
 		var sumL, sumR float64
-		for _, class := range classify.AllClasses() {
-			layer := b.layers[class]
+		for _, layer := range b.layers {
 			sample := layer.AdvanceSample()
 			gainL, gainR := PanGains(layer.Config.Pan)
-			sumL += sample * GainPerLayer * gainL
-			sumR += sample * GainPerLayer * gainR
+			sumL += sample * b.gainPerLayer * gainL
+			sumR += sample * b.gainPerLayer * gainR
 		}
 		frames[i] = [2]float64{sumL, sumR}
 	}