Function light_travel_time 

pub fn light_travel_time(distance: &Quantity) -> Option<Quantity>

The light-travel time across a distance — distance / c, exact. Returns a Time quantity, or None if distance is not a length (the dimensional guard). This is the core of “universal / space-travel time”: the delay before an event at distance d is observed elsewhere (Earth↔Sun ≈ 499 s, Earth↔Mars ≈ 3–22 min). The result rides the exact rational tower, so it never drifts.