prepare method
Builds (if needed) the morph for rest → pressed at aspect.
Implementation
KeyMorph prepare(KeyGeometry rest, KeyGeometry pressed, double aspect) {
final key = (rest, pressed, aspectKey(aspect));
final hit = _entries.remove(key);
if (hit != null) return _entries[key] = hit;
prepared++;
return _insert(key, _build(rest, pressed, key.$3 / 100));
}