Async Queue (concurrency limiter)

Build an AsyncQueue(concurrency) class that runs at most
concurrency async tasks in parallel and queues the rest.

add(task) accepts a thunk returning a Promise and returns a Promise that
resolves with the task's value once it actually runs. The queue must:

• never run more than concurrency tasks at once;
• resolve each add Promise individually as the task settles;
• not deadlock if a task throws — failed tasks must release their slot.

The harness's solve returns the **maximum number of in-flight tasks
observed** alongside the ordered results, so the test suite can verify the
concurrency cap.