[Ji-One1]

The buyer needs an observable page, and the assertion needs the active cart from that same browser session. These helpers provide both without using page text as cart proof.

What this changes

• Add a shared Playwright/Stagehand readiness predicate: DOMContentLoaded has started, and the body is visible with child elements. Page acquisition and polling share a 15-second deadline and support abort.
• Read one GET <root>cart.js in the page with the session's own cookies. Hydrogen rewrites this AJAX endpoint to upstream cart.json.
• Use the same-origin window.Shopify.routes.root, or the supplied storefront base path. Refuse unsupported roots and redirects; request no cache and bound the fetch, including the body, to five seconds.
• Copy only variant IDs, line quantities, and item count out of the page. Normalize variant IDs to ProductVariant GIDs and report failures through the five CartReadError codes defined earlier.

Developer impact

Internal tooling only; no published Hydrogen changes or changeset. No cookie is read, parsed, or set by the reader. No cart ID or Storefront API cart query is needed.

Risk

Readiness does not prove network idle, stable UI, or iframe registration. A custom storefront might serve a cart different from its UI cart at this endpoint; the helper cannot detect that. Buyer integration and CLI wiring are separate.

Stack: 10/15. Previous: #4317. Next: #4319.