seek method
Freezes the morph toward icon at t, without any spring.
Deterministic and side-effect free: it starts no ticker. Scrubbing the same target repeatedly reuses the existing plan instead of rebuilding it.
Implementation
void seek(MorphIconData icon, double t) {
if (_dead) return;
final reuse = !_rest && _plan != null && identical(icon, _target);
_stop();
_spring.v = 0; // controlled mode: no inherited velocity
if (!reuse) _retarget(icon);
_render(t);
}