Implement a Semaphore Using Futex

Implement semaphore using Linux futex system call.

Requirements:

• Use futex() system call with FUTEX_WAIT and FUTEX_WAKE
• Atomic operations with __atomic builtins
• sem_wait(): atomic decrement, block if < 0
• sem_post(): atomic increment, wake one waiter
• Handle spurious wakeups (recheck condition after futex_wait)
• Support multiple threads waiting
• Test with resource pooling scenario
• Test with producer-consumer

Success Criteria:

• Multiple threads correctly wait on semaphore
• Waking thread releases exactly one waiter
• No race conditions in counter manipulation
• Efficient: minimal kernel syscalls