Function linear_gradient 

pub fn linear_gradient(
    output: &str,
    width: u32,
    height: u32,
    angle_deg: f32,
    stops: &[ColorStop],
) -> Program

Build a Program that rasterizes a linear gradient into output.

• output: [u32; width * height] - rasterized gradient (packed RGBA)
• angle_deg: CSS angle (0 = bottom-to-top, 90 = left-to-right)
• stops: color stops (must be sorted by position, 2..=16)