update if same, else add new

cmd/cart/mutation_add_item.go

@@ -30,9 +30,13 @@ 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 {
+		if existing.StoreId == m.StoreId {
 			existing.Quantity += int(m.Quantity)
+			existing.Stock = StockStatus(m.Stock)
+			existing.StoreId = m.StoreId
 			return nil
 		}
+	}
 
 	g.mu.Lock()
 	defer g.mu.Unlock()