An async queue that preserves order

Autosave is the classic case: every keystroke can enqueue a save, the saves must not overlap or arrive out of order, and one failure must not wedge the queue for the rest of the session.

Implement a class AsyncQueue. push(task) takes a zero-argument function returning a promise, appends it to the queue, and returns a promise that settles with that task's own result. Tasks run strictly one at a time, in the order they were pushed, including tasks pushed while an earlier one is still running. If a task rejects, only the promise returned by its own push call rejects — the queue carries on with the next task.

What it has to do

• Never run two tasks at the same time.
• Start tasks in the order they were pushed, including ones pushed mid-run.
• Resolve each push with that task's own resolved value.
• Reject only the failing task's promise and keep processing the rest.

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