Block geometry/clip edits on frames that fall strictly inside a tween span,
where an edit would silently mutate the bracketing keyframe instead:
- Shape tweens: gate vector editing (vertices, curves, DCEL hits) and all
geometry tools in stage.rs behind VectorLayer::is_tween_inbetween.
- Motion tweens: block selecting/dragging/transforming clip instances whose
transform is mid-tween, via AnimationData::is_object_tweened_at.
Also: inserting a keyframe mid-tween now captures the interpolated geometry
shown at that frame (not the left keyframe's) and inherits the shape tween,
so the new keyframe continues morphing toward the right keyframe.
Skyler Lehmkuhl