feat(01-03): implement ticker-driven window aggregator with TDD

- Add Aggregate function reading ClassifiedPackets, emitting WindowSnapshots per time window
- Add DefaultWindowMs=500 constant for configurable window duration
- Flush partial window on done channel close or closed events channel
- Call optional onSnapshot callback per window for verbose mode
- Add 5 tests: EmitsSnapshot, MultipleClasses, DoneFlushesPartial, EmptyWindow, WindowIndex

aggregate/window.go

@@ -0,0 +1,63 @@
+package aggregate
+
+import (
+	"time"
+
+	"github.com/netsynth/netsynth/classify"
+)
+
+// DefaultWindowMs is the default time window duration in milliseconds.
+const DefaultWindowMs = 500
+
+// Aggregate reads ClassifiedPackets from events, accumulates counts per TrafficClass
+// in time windows of windowMs duration, and emits a WindowSnapshot per window.
+// When done is closed, flushes the current partial window and closes the output channel.
+// If onSnapshot is non-nil, it is called with each snapshot before sending to the output channel
+// (used for --verbose stderr output).
+func Aggregate(done <-chan struct{}, events <-chan classify.ClassifiedPacket,
+	windowMs int, onSnapshot func(classify.WindowSnapshot)) <-chan classify.WindowSnapshot {
+
+	out := make(chan classify.WindowSnapshot, 8)
+	go func() {
+		defer close(out)
+		ticker := time.NewTicker(time.Duration(windowMs) * time.Millisecond)
+		defer ticker.Stop()
+
+		counts := make(map[classify.TrafficClass]int64)
+		var totalPackets int64
+		windowIndex := 0
+
+		flush := func() {
+			snap := classify.WindowSnapshot{
+				Counts:       counts,
+				TotalPackets: totalPackets,
+				WindowIndex:  windowIndex,
+			}
+			if onSnapshot != nil {
+				onSnapshot(snap)
+			}
+			out <- snap
+			counts = make(map[classify.TrafficClass]int64)
+			totalPackets = 0
+			windowIndex++
+		}
+
+		for {
+			select {
+			case <-done:
+				flush() // flush final partial window
+				return
+			case <-ticker.C:
+				flush()
+			case ev, ok := <-events:
+				if !ok {
+					flush()
+					return
+				}
+				counts[ev.Class]++
+				totalPackets++
+			}
+		}
+	}()
+	return out
+}