fix: double-free in tcp_proxy_server_conn_free via freed flag

error_cb can fire twice for the same fd in one epoll batch,
or conn_free called from multiple callbacks during restart/shutdown.
freed flag makes conn_free idempotent.

src/proxy/tcp_proxy_server.c

@@ -196,6 +196,8 @@ static int conn_total(struct tcp_proxy_server_conn* rc) {
 
 void tcp_proxy_server_conn_free(struct tcp_proxy_server_conn* rc) {
     if (!rc) return;
+    if (rc->freed) return;
+    rc->freed = 1;
     int total = conn_total(rc);
     DEBUG_INFO(DEBUG_CATEGORY_SOCKET, "SOCK:FREE fd=%d sid=%08x total=%d cli_closed=%d fin=%d error=%d",
                rc->tc ? (int)rc->tc->sock : -1, rc->stream_id, total, rc->cli_closed, rc->tc ? rc->tc->fin : 0, rc->tc ? rc->tc->error : 0);

src/proxy/tcp_proxy_server.h

@@ -35,6 +35,7 @@ struct tcp_proxy_server_conn {
     void*    close_timer;      // таймер повтора CLOSE/ERROR
     int      close_backoff;    // backoff: 50..5000 tb (5ms..500ms)
     void*    diag_timer;       // 1-секундный таймер диагностики
+    uint8_t  freed;            // 1 = уже освобождён, защита от double-free
 
     struct queue_waiter_handle pause_waiter;
     struct UASYNC* ua;