Function render_border 

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

Draw the border of a box node into grid.

(x, y) is the top-left corner of the box (absolute grid coordinates). width / height are the full computed dimensions including border cells.

Mirrors renderBorder in render-border.ts:22-155, including stylePiece.