Function start_screen_capture 

pub async fn start_screen_capture(
    source_id: Option<String>,
) -> Result<(), String>

Start screen capture. source_id of None captures the primary display (M-SCK.0 default); Some("display-<id>") / Some("window-<id>") pin to a specific source (M-SCK.0.1 / AUT-291). Returns Ok(()) or Err(message).