Function serve_stdio 

pub async fn serve_stdio(
    registry: Registry,
    roster: Roster,
    root: Root,
    define: Option<DefineHandler>,
) -> Result<()>

Serve the frame loop over the process’s own stdin/stdout — the --stdio transport (docs/vscode-extension-plan.md §3.1). Exactly one client: there is no listener, so no second connection is even expressible. Returns Ok(()) when the client’s stdin reaches EOF (clean shutdown; the spawning client owns the process lifetime).