Exec, attach, and port forwarding

ExecSync runs an argv command directly in a running container with bounded output and an optional timeout. Interactive exec and attach return a short-lived single-use URL on Stackie’s host-leased loopback streaming listener after validating the requested streams and TTY mode. Port forwarding similarly binds one URL to one ready pod sandbox. A client may supply an initial unique TCP-port set or declare ports later when its upgraded logical streams open. Commands are never interpreted by a shell, and URLs are never issued for missing or stopped workloads.