Function spawn_embedded_viewer 

pub fn spawn_embedded_viewer(
    dialog_url: &str,
    options: &ViewerLaunchOptions,
) -> Result<Child, ViewerSpawnError>

Spawn wyvern-viewer for dialog_url with optional size hints.

Stdin is piped so the CLI can send exit\n after the host accepts POST /api/result (parent-controlled shutdown — page does not close the window).

Errors

Returns ViewerSpawnError when the binary is missing or spawn fails.