Multithreading with `<threads.h>` (C11)

C11 introduced a portable threading API: thrd_t, mtx_t, cnd_t etc., from <threads.h>.

Typical workflow:

1. Create a thread with thrd_create(&t, func, arg);.
2. Wait for it to finish with thrd_join(t, &ret);.
3. Protect shared data with a mutex (mtx_lock, mtx_unlock).

mtx_t lock;
mtx_init(&lock, mtx_plain);

mtx_lock(&lock);
/* critical section */
mtx_unlock(&lock);

The API is intentionally minimal – for more advanced needs use POSIX threads (pthread) on Unix or Win32 threads on Windows.