Async pool (concurrency limit)

Implement asyncPool(limit, items, fn) that:

• Runs fn(item) for every item, with at most limit running concurrently
• Resolves to an array of results in the same order as input
• If any task rejects, the whole pool rejects (fast-fail OK; bonus: cancel inflight)

await asyncPool(2, [1, 2, 3, 4], async (n) => {
  await sleep(n * 10);
  return n * 2;
});
// → [2, 4, 6, 8]