Checkpoint: Async Request Engine

This checkpoint assembles the module's pieces into the engine an API-driven app actually runs on. No browser needed — the logic is testable pure:

1. Request engine — a createLoader(fetchFn) factory: load(key) returns a promise and caches the promise by key, so concurrent duplicate requests share one flight (dedupe), and reset() clears the cache.
2. Resilient dashboard — loadDashboard(tasks) that runs all sources with allSettled and returns { loaded, failed } lists.
3. Timeout race — withTimeout(promise, ms, fallback) resolving the promise's value, or fallback if it does not settle in ms.

The shape you are building toward

Every data-driven interface reduces to this loop: event → state → render. The API supplies events (responses), your code maintains state (results, loading flag, error), and the DOM renders state. When a request is slow, the loading flag is your UI; when it fails, the error state is your UI; when the list is empty, that is a screen too.

The checkpoint asks you to wire the loop once more without the scaffold — because the next time you meet it, it will be inside a framework, and the framework will not change what the loop is. It will only change where you write each part.