StreamSourceSequencePreloaded

Server event: fires when the engine has held a source pin (pre-warmed + queued + ready). The streams list reflects the pin’s currently visible stream complement; the event re-fires whenever that complement changes so the client can wait for an expected shape (e.g. audio + video both present). The pin remains held until the client calls release_preloaded(pin) or the autopilot picks it as the next source after the current pin’s EOF.