feat(limiter/traffic): add burst support, dropped-packet counters, and cache fixes

- Add NewLimiterWithBurst(rate, burst) for independent burst control
- Extend parseLimit to accept optional 4th field as burst size
- Add DroppedPacketCounter interface with DroppedPackets() for packet/udp conns
- Export ErrRateLimited sentinel for rate-limit error checks
- Fix IP-level cache expiration (NoExpiration → defaultExpiration) to prevent
  unbounded growth; reset TTL on access in In/Out
- Short-circuit single-limiter path to avoid limiterGroup allocation
- Add nil guard in cachedTrafficLimiter when both old and new values are nil
- Remove unused ctx field from wrapper structs

limiter/traffic/generator_test.go

@@ -1,11 +1,12 @@
 package traffic
 
 import (
+	"context"
 	"testing"
 )
 
 func TestLimitGenerator_In(t *testing.T) {
-	g := newLimitGenerator(100, 0)
+	g := newLimitGenerator(100, 0, 0)
 	lim := g.In()
 	if lim == nil {
 		t.Fatal("In() should not be nil for positive in")
@@ -22,14 +23,14 @@ func TestLimitGenerator_In(t *testing.T) {
 }
 
 func TestLimitGenerator_In_Zero(t *testing.T) {
-	g := newLimitGenerator(0, 200)
+	g := newLimitGenerator(0, 200, 0)
 	if lim := g.In(); lim != nil {
 		t.Fatal("In() should be nil for zero in")
 	}
 }
 
 func TestLimitGenerator_Out(t *testing.T) {
-	g := newLimitGenerator(0, 200)
+	g := newLimitGenerator(0, 200, 0)
 	lim := g.Out()
 	if lim == nil {
 		t.Fatal("Out() should not be nil for positive out")
@@ -46,7 +47,7 @@ func TestLimitGenerator_Out(t *testing.T) {
 }
 
 func TestLimitGenerator_Out_Zero(t *testing.T) {
-	g := newLimitGenerator(100, 0)
+	g := newLimitGenerator(100, 0, 0)
 	if lim := g.Out(); lim != nil {
 		t.Fatal("Out() should be nil for zero out")
 	}
@@ -61,3 +62,27 @@ func TestLimitGenerator_NilReceiver(t *testing.T) {
 		t.Fatal("nil receiver Out() should return nil")
 	}
 }
+
+func TestLimitGenerator_Burst(t *testing.T) {
+	// Burst specified: should use NewLimiterWithBurst.
+	g := newLimitGenerator(100, 200, 500)
+	limIn := g.In()
+	if limIn == nil {
+		t.Fatal("In() should not be nil")
+	}
+	if limIn.Limit() != 100 {
+		t.Fatalf("expected limit 100, got %d", limIn.Limit())
+	}
+	// With burst=500, Wait for 500 should succeed immediately.
+	if n := limIn.Wait(context.Background(), 500); n != 500 {
+		t.Fatalf("expected 500 with burst, got %d", n)
+	}
+
+	limOut := g.Out()
+	if limOut == nil {
+		t.Fatal("Out() should not be nil")
+	}
+	if limOut.Limit() != 200 {
+		t.Fatalf("expected limit 200, got %d", limOut.Limit())
+	}
+}