Function glide_step 

pub fn glide_step(velocity: f64, seconds: f64) -> (f64, f64)

Advance a fling by seconds: the distance it covers and the velocity it keeps. The distance is the exact integral of the decay, so one 40 ms step lands where four 10 ms steps do — a fling travels the same at 25, 60 and 90 fps, and after a stalled frame. No clamp: a stall is covered, not dropped.