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

lib/socket_compat.c

@@ -89,6 +89,7 @@ int socket_set_nonblocking(socket_t sock) {
 }
 
 int socket_close_wrapper(socket_t sock) {
+    if (sock == 0) return 0;  // never close fd 0
     return close(sock);
 }