feat(02-03): implement MP3 encoder package with RunSynthesis orchestrator

- EncodeMP3: writes stereo PCM frames to MP3 via go-lame (44100 Hz, 128 kbps, stereo)
- RunSynthesis: renders WindowSnapshots through OscillatorBank then encodes to MP3
- Zero-packet guard before file creation returns descriptive error (OUT-03)
- InitParams called after all Set* calls per go-lame requirements

encode/mp3.go

@@ -0,0 +1,66 @@
+package encode
+
+import (
+	"fmt"
+	"os"
+
+	lame "github.com/sjzar/go-lame"
+
+	"github.com/netsynth/netsynth/classify"
+	"github.com/netsynth/netsynth/synth"
+)
+
+// EncodeMP3 writes stereo PCM frames to an MP3 file at the given path.
+// Encoding parameters: sampleRate Hz, 128 kbps, stereo, quality 5.
+// Per D-13 (44100 Hz CD quality) and D-14 (128 kbps bitrate).
+// CRITICAL: InitParams() is called after all Set* calls (go-lame Pitfall 1).
+func EncodeMP3(outputPath string, frames [][2]float64, sampleRate int) error {
+	f, err := os.Create(outputPath)
+	if err != nil {
+		return fmt.Errorf("create output file: %w", err)
+	}
+	defer f.Close()
+
+	wr := lame.NewWriter(f)
+	wr.Encoder.SetInSamplerate(sampleRate)
+	wr.Encoder.SetOutSamplerate(sampleRate)
+	wr.Encoder.SetNumChannels(2)
+	wr.Encoder.SetBitrate(128)
+	wr.Encoder.SetQuality(5)
+	wr.Encoder.InitParams() // MUST be called after all Set* calls
+
+	pcmBytes := synth.StereoFramesToInt16Bytes(frames)
+	if _, err := wr.Write(pcmBytes); err != nil {
+		return fmt.Errorf("write PCM to LAME encoder: %w", err)
+	}
+	if err := wr.Close(); err != nil {
+		return fmt.Errorf("finalize MP3: %w", err)
+	}
+	return nil
+}
+
+// RunSynthesis consumes a slice of WindowSnapshots, renders audio via OscillatorBank,
+// and encodes to MP3 at outputPath.
+// Returns an error if zero packets were captured (D-16 / OUT-03).
+// The zero-packet guard runs BEFORE file creation to avoid leaving an empty file on disk.
+func RunSynthesis(snapshots []classify.WindowSnapshot, outputPath string) error {
+	// D-16 / OUT-03: Zero-packet guard — check BEFORE creating output file
+	var totalPackets int64
+	for _, snap := range snapshots {
+		totalPackets += snap.TotalPackets
+	}
+	if totalPackets == 0 {
+		return fmt.Errorf("no packets captured: output MP3 not written (empty capture produces no audio)")
+	}
+
+	// Render all windows to stereo frames
+	bank := synth.NewBank(1.0) // tau=1.0s per D-07
+	var allFrames [][2]float64
+	for _, snap := range snapshots {
+		frames := bank.RenderWindow(snap)
+		allFrames = append(allFrames, frames...)
+	}
+
+	// Encode to MP3
+	return EncodeMP3(outputPath, allFrames, synth.SampleRate)
+}