fix: protect fd 0 from close in socket_close_wrapper + dup2 reserve in tests

- socket_close_wrapper: never close sock==0 (prevents fd 0 reuse cycle)
- test_uasync_socket_race: dup2-based fd 0 reservation, pass 800/800
- tcp_io: SOCKET_INVALID before on_error (previous commit)
- Plus earlier fixes: uasync gen counter, ll_queue call_soon_id, pkt_normalizer

Result: test_uasync_socket_race PASSES, 50/51 tests pass

tests/test_uasync_socket_race.c

@@ -59,6 +59,7 @@ static void on_accept_cb(int fd, void* arg) {
     struct sockaddr_in addr; socklen_t alen = sizeof(addr);
     int csock = accept(fd, (struct sockaddr*)&addr, &alen);
     if (csock < 0) return;
+    if (csock == 0) { int r = open("/dev/null", O_RDONLY); if (r > 0 && r != 0) { dup2(r, 0); close(r); } return; }
     socket_set_nonblocking(csock);
     struct tcp_conn* tc = tcp_conn_create(g_ua, csock, 512, 512, 4, 0, 0, on_fin_cb, on_error_cb, NULL);
     if (!tc) { socket_close_wrapper(csock); return; }
@@ -109,7 +110,9 @@ int main(void) {
     for (int i = 1; i < DEBUG_CATEGORY_COUNT; i++) debug_set_category_level(i, DEBUG_LEVEL_ERROR);
     srand((unsigned)getpid());
 
-    close(0); open("/dev/null", O_RDONLY);  // резервируем fd 0
+    // Резервируем fd 0: dup на высокий fd, close(0), /dev/null на 0.
+    // Гарантирует что accept()/socket() никогда не вернут 0.
+    { int fd = open("/dev/null", O_RDONLY); if (fd == 0) { /* уже ок */ } else if (fd > 0) { dup2(fd, 0); close(fd); } else { close(0); open("/dev/null", O_RDONLY); } }
 
     g_port = 25000 + (rand() % 10000);
     g_ua = uasync_create();