Disable Purchases for Logged-Out Users

How can you disable purchases for logged-out users in WordPress? This use case shows how a focused WordPress code snippet can handle the task without installing a heavy plugin. The snippet should disable add-to-cart for logged-out users and produce this result: Visitors must log in before they can add products to the cart. You can also log in to LeonLab.ai, describe this WordPress use case in plain English, generate the snippet, review the PHP code, and deploy it to the selected WordPress project.