Fix flaky NATSEventBus integration tests

The integration tests had timing issues causing intermittent failures on CI:

- TestNATSEventBus_HighThroughput: Added subscriber readiness synchronization using a barrier event before bulk publishing. This ensures the NATS subscription is fully established before events are sent rapidly. Extended timeout from 30s to 60s for CI environments.

- TestNATSEventBus_EventOrdering: Added readiness barrier event to synchronize subscriber setup before publishing ordered events. Extended timeout from 10s to 15s to account for CI timing variations.

- TestNATSEventBus_ConcurrentPublishSubscribe: Added readiness synchronization before concurrent publishers start. Extended timeout from 10s to 30s to handle the increased load under CI constraints.

Root causes:
- Subscriber channels were not fully ready to receive when bulk publishing started, causing message loss
- CI runners (especially ARM64) have different timing characteristics than local development
- Insufficient timeouts for high-volume event collection under shared CI resources

The fixes use a barrier pattern: publish a ready signal, wait to receive it, then proceed with the test. This is more reliable than fixed sleep durations.

Closes #57

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

nats_eventbus_integration_test.go

@@ -281,7 +281,25 @@ func TestNATSEventBus_HighThroughput(t *testing.T) {
 	ch := bus.Subscribe(namespace)
 	defer bus.Unsubscribe(namespace, ch)
 
-	time.Sleep(100 * time.Millisecond)
+	// Synchronize subscriber readiness using a barrier event.
+	// This ensures the NATS subscription is fully established before bulk publishing.
+	readyEvent := &Event{
+		ID:        "ready-signal",
+		EventType: "ReadySignal",
+		ActorID:   "actor-throughput",
+		Version:   0,
+		Data:      map[string]interface{}{},
+		Timestamp: time.Now(),
+	}
+	bus.Publish(namespace, readyEvent)
+
+	// Wait for the ready signal to be received
+	select {
+	case <-ch:
+		// Subscriber is ready, proceed with bulk publishing
+	case <-time.After(5 * time.Second):
+		t.Fatal("timeout waiting for subscriber readiness signal")
+	}
 
 	// Publish many events rapidly
 	start := time.Now()
@@ -298,10 +316,10 @@ func TestNATSEventBus_HighThroughput(t *testing.T) {
 	}
 	publishDuration := time.Since(start)
 
-	// Receive events with timeout
+	// Receive events with extended timeout for CI environment
 	receivedCount := 0
 	receiveStart := time.Now()
-	timeout := time.After(30 * time.Second)
+	timeout := time.After(60 * time.Second)
 
 loop:
 	for receivedCount < numEvents {
@@ -339,7 +357,23 @@ func TestNATSEventBus_EventOrdering(t *testing.T) {
 	ch := bus.Subscribe(namespace)
 	defer bus.Unsubscribe(namespace, ch)
 
-	time.Sleep(100 * time.Millisecond)
+	// Synchronize subscriber readiness with barrier event
+	readyEvent := &Event{
+		ID:        "ordering-ready",
+		EventType: "ReadySignal",
+		ActorID:   "actor-ordering",
+		Version:   0,
+		Data:      map[string]interface{}{},
+		Timestamp: time.Now(),
+	}
+	bus.Publish(namespace, readyEvent)
+
+	select {
+	case <-ch:
+		// Subscriber is ready
+	case <-time.After(5 * time.Second):
+		t.Fatal("timeout waiting for ordering readiness signal")
+	}
 
 	// Publish events with sequence numbers
 	for i := 0; i < numEvents; i++ {
@@ -356,7 +390,7 @@ func TestNATSEventBus_EventOrdering(t *testing.T) {
 
 	// Receive and verify ordering
 	received := make([]*Event, 0, numEvents)
-	timeout := time.After(10 * time.Second)
+	timeout := time.After(15 * time.Second)
 
 loop:
 	for len(received) < numEvents {
@@ -487,7 +521,23 @@ func TestNATSEventBus_ConcurrentPublishSubscribe(t *testing.T) {
 	ch := bus.Subscribe(namespace)
 	defer bus.Unsubscribe(namespace, ch)
 
-	time.Sleep(100 * time.Millisecond)
+	// Synchronize publisher readiness
+	readyEvent := &Event{
+		ID:        "concurrent-ready",
+		EventType: "ReadySignal",
+		ActorID:   "coordinator",
+		Version:   0,
+		Data:      map[string]interface{}{},
+		Timestamp: time.Now(),
+	}
+	bus.Publish(namespace, readyEvent)
+
+	select {
+	case <-ch:
+		// Subscriber is ready
+	case <-time.After(5 * time.Second):
+		t.Fatal("timeout waiting for concurrent readiness signal")
+	}
 
 	var wg sync.WaitGroup
 	wg.Add(numPublishers)
@@ -513,9 +563,9 @@ func TestNATSEventBus_ConcurrentPublishSubscribe(t *testing.T) {
 	// Wait for all publishers to finish
 	wg.Wait()
 
-	// Count received events
+	// Count received events with extended timeout for CI environment
 	receivedCount := 0
-	timeout := time.After(10 * time.Second)
+	timeout := time.After(30 * time.Second)
 
 loop:
 	for receivedCount < totalExpected {