Function render 

pub fn render(
    level: Level,
    rect: Rect,
    strip_buf: &mut Vec<Strip>,
    alpha_buf: &mut Vec<u8>,
)

Render a pixel-aligned rectangle directly into strips.

This bypasses the full path processing pipeline (flatten → tiles → strips) by directly creating strip coverage data for the rectangle.

The rect bounds should already be clamped to the viewport.