missing updates (#5)

Co-authored-by: matst80 <mats.tornberg@gmail.com>
Reviewed-on: https://git.tornberg.me/mats/go-cart-actor/pulls/5
Co-authored-by: Mats Törnberg <mats@tornberg.me>
Co-committed-by: Mats Törnberg <mats@tornberg.me>

cmd/cart/mutation_add_item.go

@@ -30,8 +30,12 @@ func AddItem(g *CartGrain, m *messages.AddItem) error {
 
 	// Fast path: merge with existing item having same SKU
 	if existing, found := g.FindItemWithSku(m.Sku); found {
-		existing.Quantity += int(m.Quantity)
-		return nil
+		if existing.StoreId == m.StoreId {
+			existing.Quantity += int(m.Quantity)
+			existing.Stock = StockStatus(m.Stock)
+			existing.StoreId = m.StoreId
+			return nil
+		}
 	}
 
 	g.mu.Lock()