Function render_kitty 

pub fn render_kitty(spec: &ChartSpec, opts: KittyOpts) -> String

Render spec to a PNG and return the kitty APC escape string that displays it at the current cursor.

Returns an empty string when drawing fails or when PNG encoding fails. A blank chart is the least intrusive fallback: the TUI’s chart pane will simply show nothing rather than garbling the terminal with malformed control bytes.