A single declared variable: the value it holds before any keyframe
fires (and its whole value, forever, if animation is empty — see
VarDef::is_static), plus an ordered list of keyframes that tween it
across the scenario’s absolute timeline.
One tween: hold at the running value until at, then ease to to over
duration, then hold at to until the next keyframe fires (or
forever, for the last one). See super::track’s module doc, “Value
outside the keyframes”, for the exact before-first/after-last rule.