Function render_background 

pub fn render_background(
    x: i32,
    y: i32,
    width: u16,
    height: u16,
    style: &Style,
    grid: &mut Grid,
    level: ColorLevel,
)

Draw a box node’s backgroundColor rectangle FILL into grid.

(x, y) is the box top-left (absolute grid coords); width / height are the full computed dimensions INCLUDING border cells. No-op when the node has no backgroundColor, or when borders consume the entire content area (contentWidth <= 0 || contentHeight <= 0). Port of render-background.ts.

level is the detected color level: at ColorLevel::None the fill line’s colorize is a no-op, so the spaces carry no SGR (they then trim away as blank pad) — matching ink/chalk in a non-color terminal.