Semaphore

Implement createSemaphore(permits) that controls concurrent access to a shared resource.

Requirements

• acquire() — returns a Promise that resolves when a permit is available. Decrements the permit count.
• release() — frees one permit, allowing a waiting acquirer to proceed.
• If permits are available, acquire() resolves immediately.
• If no permits are available, acquire() waits until one is released.

Example

const sem = createSemaphore(2);
await sem.acquire(); // resolves immediately (permits: 1)
await sem.acquire(); // resolves immediately (permits: 0)
sem.acquire();       // waits... (no permits)
sem.release();       // now the waiting acquire resolves