Function spring_rest_time 

pub fn spring_rest_time(config: &SpringConfig) -> f64

Public “measure du repos” (issue #167 lot E): the instant, in seconds, at which this spring settles within rest_threshold of its target and stays there — what rustmotion info surfaces so an author can size a scene/preset duration around a spring instead of discovering it by trial and error.

When config.duration is set, this is that duration, exactly — that is the point of the time remap spring_value performs (see its doc comment). Otherwise it is the natural settle time computed from damping/stiffness/mass alone via spring_settle_time.