Function launch_with 

pub fn launch_with<P: Platform>(
    backend: P,
    options: WindowOptions,
    root_piece: impl FnOnce() -> AnyPiece + 'static,
)

Launch a Day app on the given platform backend: sets up the reactive scheduler and the cross-thread poster, mounts the root piece into the window’s content container, runs the initial layout, and installs the turn-end layout callback (§3.3). The backend then owns the native main loop.