feat(02-03): wire -o output flag and snapshot accumulator stub into CLI

- Add outputPath var and -o/--output Cobra flag (OUT-01)
- Default output path resolves to netsynth-<timestamp>.mp3 when -o not provided (D-15)
- Snapshot accumulator collects all WindowSnapshots for Phase 3 synthesis wiring
- TODO(phase-3) marker for encode.RunSynthesis integration

cmd/netsynth/main.go

@@ -8,6 +8,7 @@ import (
 	"strings"
 	"sync/atomic"
 	"syscall"
+	"time"
 
 	"github.com/spf13/cobra"
 
@@ -20,6 +21,7 @@ var (
 	ifaceName  string
 	listIfaces bool
 	verbose    bool
+	outputPath string
 )
 
 func main() {
@@ -33,6 +35,7 @@ func main() {
 	rootCmd.Flags().StringVarP(&ifaceName, "interface", "i", "", "Network interface to capture on (required unless --list-interfaces)")
 	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)")
 
 	if err := rootCmd.Execute(); err != nil {
 		os.Exit(1)
@@ -45,6 +48,11 @@ func run(cmd *cobra.Command, args []string) error {
 		return runListInterfaces()
 	}
 
+	// Resolve default output path (D-15 / OUT-01)
+	if outputPath == "" {
+		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")
@@ -80,12 +88,17 @@ func run(cmd *cobra.Command, args []string) error {
 	}
 	snapshots := aggregate.Aggregate(ctx.Done(), classified, aggregate.DefaultWindowMs, onSnapshot)
 
-	// Consume snapshots and accumulate totals
+	// Accumulate snapshots for synthesis (Phase 3 will pass to encode.RunSynthesis)
 	totals := make(map[classify.TrafficClass]int64)
+	var collectedSnapshots []classify.WindowSnapshot
 	for snap := range snapshots {
+		collectedSnapshots = append(collectedSnapshots, snap)
 		aggregate.AccumulateTotals(totals, snap)
 	}
 
+	// TODO(phase-3): Pass collectedSnapshots to encode.RunSynthesis(collectedSnapshots, outputPath)
+	_ = collectedSnapshots
+
 	// Print exit summary (CLAS-03)
 	dropped := atomic.LoadInt64(droppedPtr)
 	if dropped > 0 {