Function merge_curve

pub fn merge_curve(
    preset: &CurvePreset,
    anim: &mut Animated,
    span: f64,
    offset: f64,
)

apply_curve’s sibling: ADD the preset’s keys to what the property already has, shifted to start at offset seconds, so two presets can be layered on one property without hand-editing. span is the length a normalised preset is stretched over (callers pass the clip’s remaining time so the merged keys land inside it). A preset key falling on an existing key’s time wins.