Allow automatic split for same product: sell available stock first and treat remaining quantity as preorder with proper messaging

When a product has limited stock (e.g., 1 unit), customers can still buy multiple units without any preorder message. need mixed behavior where available stock is sold first, and any additional quantity is automatically treated as preorder. The storefront should clearly inform customers (for example: “1 item ships now, remaining items is a preorder and will ship later”).