102Implement a task scheduler with concurrency limits, priorities, cancellation, and starvation prevention

Implement class Scheduler:

const scheduler = new Scheduler({ concurrency: 2, agingMs: 1000, now: Date.now });
const result = await scheduler.schedule(({ signal }) => fetch(url, { signal }), { priority: 5, signal });

• schedule(task, { priority = 0, signal } = {}) returns a promise for the task's result. task receives { signal }.
• At most concurrency tasks run at once. When a slot frees, the waiting task with the highest effective priority starts.
• Starvation prevention (aging): effective priority = priority + waitedMs / agingMs. A low-priority task that has waited long enough eventually beats newly scheduled high-priority tasks. Ties go to the task scheduled first.
• Cancellation: if signal aborts while the task is waiting, remove it and reject with signal.reason without running it. If signal is already aborted, reject immediately. If it aborts while the task is running, the task sees it through its own signal (pass it on to fetch and friends); the scheduler rejects with signal.reason at once and frees the slot.
• size (waiting) and running counts.

now is injectable so tests can control time.