Guard async request maps with a load generation, not just a timeout

When a single-page viewer sends a request to the native side and keeps {id -> context, timer} in a Map with a setTimeout fallback toast, deleting the entry inside the timer callback (if (map.delete(id)) toast()) makes clearTimeout redundant and prevents double toasts or late answers. The deletion does not cover the page loading another document: the timer still fires on the new document, and a late answer is applied to a stale row id. Store the current load generation in the entry and drop it silently in both the timer and the answer handler when the generation has changed. Mutation testing helps tell guards apart: replacing delete with has fails the late-answer check, while removing clearTimeout passes, which shows it is only defensive.