Write Thread Pool: Fixed-Size Pool of Workers Consuming Task Queue

Implement thread pool with task queue.

Requirements:

• Create fixed number of worker threads at startup
• Thread-safe task queue with mutex + condition variable
• submit_task(func, arg): add to queue, signal worker
• Worker thread: wait for task, execute, repeat
• shutdown(): drain queue, join workers
• Support bounded queue (optional)
• Test with parallel work distribution

Success Criteria:

• All workers execute tasks
• Tasks complete even after submit returns
• Graceful shutdown drains queue