UAT browser check

Reproduce a user-visible behaviour in a separate headless Chrome, read the DOM as evidence and send screenshots for approval. The library is scripts/cdp.mjs (Node >= 22, global fetch and WebSocket); scenarios are small scripts written per check.

When to use it

• A UAT item needs a browser: service worker lifecycle, update notice, offline or reconnect, two tabs, theme contrast, a layout at a declared viewport.
• The user tried a manual step and it failed or was inconclusive: reproduce before touching code, instrumenting each event, so the report separates "the code works" from "the test procedure missed".
• A visual decision needs the user's approval: screenshot it instead of describing it.

Do not use it to replace real hardware checks (touch targets with the finger, system font scaling on the tablet, cabin lighting). Those stay as backstops marked blocked when there is no device.

Preconditions