Function vertical_fade 

pub fn vertical_fade(
    scene: &mut Scene,
    transform: Affine,
    rect: Rect,
    edge: FadeEdge,
    strength: f32,
    theme: &Theme,
)

Fade rect from media.scrim at strength on edge to clear on the opposite edge, so text drawn on media near that edge (in media.ink) stays legible. One smooth gradient, eased so the fade has no visible start line; never stepped bands. strength is clamped to MIN_FADE_STRENGTH..=1; zero paints nothing.