server: gate UVC SET_CUR ack on env

server/src/bin/lesavka-uvc.rs

@@ -524,8 +524,9 @@ fn handle_data(
                     payload
                 );
             }
-            // ACK commit after we've processed the payload.
-            let _ = send_response(fd, uvc_send_response, &[]);
+            if std::env::var("LESAVKA_UVC_ACK_AFTER_DATA").is_ok() {
+                let _ = send_response(fd, uvc_send_response, &[]);
+            }
         }
     }
 }