fix: replace usleep with nanosleep for POSIX compliance

Co-authored-by: aider (deepseek/deepseek-coder) <aider@aider.chat>

engine.c

@@ -370,7 +370,8 @@ void* save_load_thread_func(void *arg) {
             }
         } else {
             // No requests, sleep a bit to avoid busy-waiting
-            usleep(1000);  // 1ms
+            struct timespec ts = { .tv_sec = 0, .tv_nsec = 1000000 };  // 1ms
+            nanosleep(&ts, NULL);
         }
     }