Class AsyncQueue<T>

A generic, single-consumer async queue that implements AsyncIterable, bridging a push producer to a pull consumer (for await (const x of q)).

Producers call push as values are produced and close (or fail) when finished. A single consumer drains the queue.

Semantics:

• Buffered items are always delivered before an end/error signal.
• close ends iteration cleanly (done: true); idempotent.
• fail surfaces the error to the consumer after any buffered items have been drained; it is sticky (first failure wins) and also closes the queue, so a later close() can't discard the error.
• push after close/fail is ignored (the producer has already signalled completion).