Function split 

pub fn split<E: Serialize>(
    primary: Widget,
    detail: Widget,
    show_detail: bool,
    on_back: E,
) -> Widget

A two-pane master-detail layout (Widget::Split). Side-by-side on a wide screen (tablet / landscape); one pane on a phone — primary until show_detail (the app sets it on selection), then detail with a back chevron firing on_back. On wide, detail should show a placeholder until a row is selected.