Function render_spec 

pub fn render_spec(
    spec: &WidgetSpec,
    prev: &HashMap<String, WidgetInstanceState>,
    prev_focus_key: &str,
    panel_width: u32,
) -> RenderOutput

Render a spec to a RenderOutput.

prev is the previous render’s instance state (or empty on first mount). prev_focus_key is the previous render’s focus key (or ""); the renderer keeps it if it matches a tabbable in the new spec, otherwise falls back to the first tabbable. panel_width is the buffer’s column width — used by Row to size flex Spacers. Pass u32::MAX to disable flex (children won’t be padded).