fix: use _Atomic flag for sel_alloc_thread_id registration, move reg before loop, add wrong-thread guard test

src/scheduler/src/main.c

@@ -117,16 +117,11 @@ int main(int argc, char* argv[])
 
     INFO("xce scheduler is running");
 
+    /* Register this thread as the sole eval/GC allocator thread (write-once) */
+    sel_register_alloc_thread();
+
     while (1) {
         sched_update(&sched);
-
-        /* Register this thread as the sole eval/GC allocator thread (write-once) */
-        static int sel_thread_registered = 0;
-        if (!sel_thread_registered) {
-            sel_register_alloc_thread();
-            sel_thread_registered = 1;
-        }
-
         sched_begin_session(&sched);
         sched_go(&sched);