feat(04-02): wire --filter and --read flags into CLI with branching run logic

- Add bpfFilter and readPath global vars with flag registration
- Add deriveOutputPath helper: capture.pcap -> capture.mp3 (D-05)
- run() validates mutual exclusion of --read and -i (D-03)
- run() pre-validates BPF filter via capture.ValidateBPFFilter before capture (CAPT-05)
- runLiveMode: extracts existing live pipeline, passes bpfFilter to StartCapture
- runPcapMode: reads pcap with ReadPcapFile, classifies with timestamps, AggregatePcap, encodes
- Bookend messages: 'Reading <file>...' at start, 'Saved...' at end (D-06)
- --verbose supported in pcap mode via onSnapshot callback (D-07)
- Empty pcap produces clear error with file path (Pitfall 4)

cmd/netsynth/main.go

@@ -5,6 +5,7 @@ import (
 	"fmt"
 	"os"
 	"os/signal"
+	"path/filepath"
 	"strings"
 	"sync/atomic"
 	"syscall"
@@ -23,6 +24,8 @@ var (
 	listIfaces bool
 	verbose    bool
 	outputPath string
+	bpfFilter  string // NEW: --filter flag (CAPT-05)
+	readPath   string // NEW: --read flag (CAPT-06)
 )
 
 func main() {
@@ -37,35 +40,69 @@ func main() {
 	rootCmd.Flags().BoolVar(&listIfaces, "list-interfaces", false, "List available network interfaces and exit")
 	rootCmd.Flags().BoolVar(&verbose, "verbose", false, "Print per-window protocol activity to stderr")
 	rootCmd.Flags().StringVarP(&outputPath, "output", "o", "", "Output MP3 file path (default: netsynth-<timestamp>.mp3)")
+	rootCmd.Flags().StringVar(&bpfFilter, "filter", "", "BPF filter expression (tcpdump syntax, e.g. \"port 53\")")
+	rootCmd.Flags().StringVar(&readPath, "read", "", "Read packets from pcap file instead of live capture")
 
 	if err := rootCmd.Execute(); err != nil {
 		os.Exit(1)
 	}
 }
 
+// deriveOutputPath replaces the file extension with .mp3 per D-05.
+// "capture.pcap" -> "capture.mp3", "noext" -> "noext.mp3"
+func deriveOutputPath(rPath string) string {
+	ext := filepath.Ext(rPath)
+	if ext == "" {
+		return rPath + ".mp3"
+	}
+	return strings.TrimSuffix(rPath, ext) + ".mp3"
+}
+
 func run(cmd *cobra.Command, args []string) error {
 	// --list-interfaces mode (CAPT-02)
 	if listIfaces {
 		return runListInterfaces()
 	}
 
-	// Resolve default output path (D-15 / OUT-01)
+	// D-03: --read and -i are mutually exclusive
+	if readPath != "" && ifaceName != "" {
+		return fmt.Errorf("--read and -i are mutually exclusive; use one or the other")
+	}
+	if readPath == "" && ifaceName == "" {
+		return fmt.Errorf("interface required: use -i <interface>, --read <file>, or --list-interfaces to see available interfaces")
+	}
+
+	// Pre-validate BPF filter before any capture (CAPT-05, Pitfall 1)
+	if bpfFilter != "" {
+		if err := capture.ValidateBPFFilter(bpfFilter); err != nil {
+			return err
+		}
+	}
+
+	// Resolve output path
 	if outputPath == "" {
+		if readPath != "" {
+			outputPath = deriveOutputPath(readPath) // D-05
+		} else {
 			outputPath = fmt.Sprintf("netsynth-%s.mp3", time.Now().Format("20060102-150405"))
 		}
-
-	// Require -i flag
-	if ifaceName == "" {
-		return fmt.Errorf("interface required: use -i <interface> or --list-interfaces to see available interfaces")
 	}
 
+	if readPath != "" {
+		return runPcapMode(cmd)
+	}
+	return runLiveMode(cmd)
+}
+
+// runLiveMode runs the live packet capture pipeline.
+func runLiveMode(cmd *cobra.Command) error {
 	// Set up signal handling (Ctrl+C)
 	ctx, stop := signal.NotifyContext(context.Background(), os.Interrupt, syscall.SIGTERM)
 	defer stop()
 
-	// Stage 1: Capture (CAPT-01)
+	// Stage 1: Capture (CAPT-01) with optional BPF filter (CAPT-05)
 	fmt.Fprintf(os.Stderr, "Starting capture on %s... (press Ctrl+C to stop)\n", ifaceName)
-	packets, droppedPtr, err := capture.StartCapture(ctx, ifaceName, "")
+	packets, droppedPtr, err := capture.StartCapture(ctx, ifaceName, bpfFilter)
 	if err != nil {
 		return err // CAPT-04: permission error already has platform-specific message
 	}
@@ -89,7 +126,7 @@ func run(cmd *cobra.Command, args []string) error {
 	}
 	snapshots := aggregate.Aggregate(ctx.Done(), classified, aggregate.DefaultWindowMs, onSnapshot)
 
-	// Accumulate snapshots for synthesis (Phase 3 will pass to encode.RunSynthesis)
+	// Accumulate snapshots for synthesis
 	totals := make(map[classify.TrafficClass]int64)
 	var collectedSnapshots []classify.WindowSnapshot
 	for snap := range snapshots {
@@ -123,6 +160,74 @@ func run(cmd *cobra.Command, args []string) error {
 	return nil
 }
 
+// runPcapMode runs the pcap file processing pipeline (CAPT-06).
+func runPcapMode(cmd *cobra.Command) error {
+	// D-06: bookend start message
+	fmt.Fprintf(os.Stderr, "Reading %s...\n", readPath)
+
+	// Open pcap file with optional software BPF filter (D-04)
+	packets, err := capture.ReadPcapFile(readPath, bpfFilter)
+	if err != nil {
+		return err
+	}
+
+	// Classify packets (reuse same classifier)
+	classifier := classify.NewClassifier(classify.DefaultRules)
+	classified := make(chan classify.ClassifiedPacket, 1024)
+	go func() {
+		defer close(classified)
+		for pkt := range packets {
+			cp := classifier.Classify(pkt)
+			// Set timestamp from pcap metadata for D-01 window assignment
+			cp.Timestamp = pkt.Metadata().CaptureInfo.Timestamp
+			classified <- cp
+		}
+	}()
+
+	// D-07: --verbose callback (same as live mode)
+	var onSnapshot func(classify.WindowSnapshot)
+	if verbose {
+		onSnapshot = func(snap classify.WindowSnapshot) {
+			aggregate.PrintWindowLine(os.Stderr, snap)
+		}
+	}
+
+	// Timestamp-based aggregation (D-01, D-02)
+	collectedSnapshots := aggregate.AggregatePcap(classified, aggregate.DefaultWindowMs, onSnapshot)
+
+	// Accumulate totals for summary
+	totals := make(map[classify.TrafficClass]int64)
+	for _, snap := range collectedSnapshots {
+		aggregate.AccumulateTotals(totals, snap)
+	}
+
+	// D-06: protocol summary (same format as live mode)
+	aggregate.PrintSummary(os.Stderr, totals)
+
+	// Check for empty pcap (Pitfall 4: better error than generic "no packets captured")
+	if len(collectedSnapshots) == 0 {
+		return fmt.Errorf("pcap file %q contains no packets (after filtering)", readPath)
+	}
+
+	// Encode
+	fmt.Fprintf(os.Stderr, "Encoding %d windows to %s...\n", len(collectedSnapshots), outputPath)
+	encodeStart := time.Now()
+	if err := encode.RunSynthesis(collectedSnapshots, outputPath); err != nil {
+		return fmt.Errorf("synthesis failed: %w", err)
+	}
+	encodeElapsed := time.Since(encodeStart)
+
+	// D-06: bookend end message
+	audioDuration := float64(len(collectedSnapshots)) * float64(aggregate.DefaultWindowMs) / 1000.0
+	info, statErr := os.Stat(outputPath)
+	if statErr != nil {
+		return fmt.Errorf("stat output file: %w", statErr)
+	}
+	fmt.Fprintf(os.Stderr, "Saved %s (%.1fs, %d KB, encoded in %.1fs)\n",
+		outputPath, audioDuration, info.Size()/1024, encodeElapsed.Seconds())
+	return nil
+}
+
 func runListInterfaces() error {
 	ifaces, err := capture.ListInterfaces()
 	if err != nil {