feat(event-sourcing): Publish EventStored after successful SaveEvent

Add EventStored internal event published to the EventBus when events are
successfully persisted. This allows observability components (metrics,
projections, audit systems) to react to persisted events without coupling
to application code.

Implementation:
- Add EventTypeEventStored constant to define the event type
- Update InMemoryEventStore with optional EventBroadcaster support
- Add NewInMemoryEventStoreWithBroadcaster constructor
- Update JetStreamEventStore with EventBroadcaster support
- Add NewJetStreamEventStoreWithBroadcaster constructor
- Implement publishEventStored() helper method
- Publish EventStored containing EventID, ActorID, Version, Timestamp
- Only publish on successful SaveEvent (not on version conflicts)
- Automatically recorded in metrics through normal Publish flow

Test coverage:
- EventStored published after successful SaveEvent
- No EventStored published on version conflict
- Multiple EventStored events published in order
- SaveEvent works correctly without broadcaster (nil-safe)

Closes #61

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

store/memory_test.go

@@ -1905,3 +1905,181 @@ func TestSaveEvent_MetadataPreservedAcrossMultipleEvents(t *testing.T) {
 		}
 	}
 }
+
+// === EventStored Publishing Tests ===
+
+func TestSaveEvent_WithBroadcaster_PublishesEventStored(t *testing.T) {
+	// Create a mock broadcaster to capture published events
+	broadcaster := aether.NewEventBus()
+	store := NewInMemoryEventStoreWithBroadcaster(broadcaster, "test-namespace")
+
+	// Subscribe to EventStored events
+	ch := broadcaster.Subscribe("test-namespace")
+	defer broadcaster.Unsubscribe("test-namespace", ch)
+
+	event := &aether.Event{
+		ID:        "evt-123",
+		EventType: "OrderPlaced",
+		ActorID:   "order-456",
+		Version:   1,
+		Data: map[string]interface{}{
+			"total": 100.50,
+		},
+		Timestamp: time.Now(),
+	}
+
+	// Save event
+	err := store.SaveEvent(event)
+	if err != nil {
+		t.Fatalf("SaveEvent failed: %v", err)
+	}
+
+	// Check if EventStored was published
+	select {
+	case publishedEvent := <-ch:
+		if publishedEvent == nil {
+			t.Fatal("received nil event from broadcaster")
+		}
+		if publishedEvent.EventType != aether.EventTypeEventStored {
+			t.Errorf("expected EventType %q, got %q", aether.EventTypeEventStored, publishedEvent.EventType)
+		}
+		if publishedEvent.ActorID != "order-456" {
+			t.Errorf("expected ActorID %q, got %q", "order-456", publishedEvent.ActorID)
+		}
+		if publishedEvent.Version != 1 {
+			t.Errorf("expected Version 1, got %d", publishedEvent.Version)
+		}
+		// Check data contains original event info
+		if publishedEvent.Data["eventId"] != "evt-123" {
+			t.Errorf("expected eventId %q, got %q", "evt-123", publishedEvent.Data["eventId"])
+		}
+	case <-time.After(100 * time.Millisecond):
+		t.Fatal("timeout waiting for EventStored event")
+	}
+}
+
+func TestSaveEvent_VersionConflict_NoEventStored(t *testing.T) {
+	broadcaster := aether.NewEventBus()
+	store := NewInMemoryEventStoreWithBroadcaster(broadcaster, "test-namespace")
+
+	// Subscribe to EventStored events
+	ch := broadcaster.Subscribe("test-namespace")
+	defer broadcaster.Unsubscribe("test-namespace", ch)
+
+	// Save first event
+	event1 := &aether.Event{
+		ID:        "evt-1",
+		EventType: "OrderPlaced",
+		ActorID:   "order-456",
+		Version:   1,
+		Data:      map[string]interface{}{},
+		Timestamp: time.Now(),
+	}
+
+	err := store.SaveEvent(event1)
+	if err != nil {
+		t.Fatalf("SaveEvent(event1) failed: %v", err)
+	}
+
+	// Drain the first EventStored event
+	select {
+	case <-ch:
+	case <-time.After(100 * time.Millisecond):
+		t.Fatal("timeout waiting for first EventStored event")
+	}
+
+	// Try to save event with non-increasing version (should fail)
+	event2 := &aether.Event{
+		ID:        "evt-2",
+		EventType: "OrderPlaced",
+		ActorID:   "order-456",
+		Version:   1, // Same version, should conflict
+		Data:      map[string]interface{}{},
+		Timestamp: time.Now(),
+	}
+
+	err = store.SaveEvent(event2)
+	if !errors.Is(err, aether.ErrVersionConflict) {
+		t.Fatalf("expected ErrVersionConflict, got %v", err)
+	}
+
+	// Verify no EventStored event was published
+	select {
+	case <-ch:
+		t.Fatal("expected no EventStored event, but received one")
+	case <-time.After(50 * time.Millisecond):
+		// Expected - no event published
+	}
+}
+
+func TestSaveEvent_MultipleEvents_PublishesMultipleEventStored(t *testing.T) {
+	broadcaster := aether.NewEventBus()
+	store := NewInMemoryEventStoreWithBroadcaster(broadcaster, "test-namespace")
+
+	// Subscribe to EventStored events
+	ch := broadcaster.Subscribe("test-namespace")
+	defer broadcaster.Unsubscribe("test-namespace", ch)
+
+	// Save multiple events
+	for i := int64(1); i <= 3; i++ {
+		event := &aether.Event{
+			ID:        fmt.Sprintf("evt-%d", i),
+			EventType: "OrderPlaced",
+			ActorID:   "order-456",
+			Version:   i,
+			Data:      map[string]interface{}{},
+			Timestamp: time.Now(),
+		}
+
+		err := store.SaveEvent(event)
+		if err != nil {
+			t.Fatalf("SaveEvent failed: %v", err)
+		}
+	}
+
+	// Verify we received 3 EventStored events in order
+	for i := int64(1); i <= 3; i++ {
+		select {
+		case publishedEvent := <-ch:
+			if publishedEvent == nil {
+				t.Fatal("received nil event from broadcaster")
+			}
+			if publishedEvent.Version != i {
+				t.Errorf("expected Version %d, got %d", i, publishedEvent.Version)
+			}
+		case <-time.After(100 * time.Millisecond):
+			t.Fatalf("timeout waiting for EventStored event %d", i)
+		}
+	}
+}
+
+func TestSaveEvent_WithoutBroadcaster_NoPanic(t *testing.T) {
+	// Test that SaveEvent works without a broadcaster (nil broadcaster)
+	store := NewInMemoryEventStore()
+
+	event := &aether.Event{
+		ID:        "evt-123",
+		EventType: "OrderPlaced",
+		ActorID:   "order-456",
+		Version:   1,
+		Data: map[string]interface{}{
+			"total": 100.50,
+		},
+		Timestamp: time.Now(),
+	}
+
+	// This should not panic even though broadcaster is nil
+	err := store.SaveEvent(event)
+	if err != nil {
+		t.Fatalf("SaveEvent failed: %v", err)
+	}
+
+	// Verify event was saved
+	events, err := store.GetEvents("order-456", 0)
+	if err != nil {
+		t.Fatalf("GetEvents failed: %v", err)
+	}
+	if len(events) != 1 {
+		t.Fatalf("expected 1 event, got %d", len(events))
+	}
+}