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.
`tween_after == Shape` was stored on keyframes but never read. Now the
render path morphs geometry across a shape-tween span:
- VectorGraph::interpolated(other, t): same-topology lerp of vertex
positions, edge curves, stroke widths and stroke/fill colours. Returns
None when topology differs (counts, deleted flags, edge endpoints, fill
boundaries), so the caller holds the source keyframe.
- VectorLayer::tweened_graph_at(time): returns an owned morphed graph for
a shape-tween span whose two keyframes share topology, else borrows the
held keyframe. Editing still uses graph_at_time (the held keyframe).
- Renderer (Vello + CPU paths) renders via tweened_graph_at.
- SetTweenAction + wired the previously-stubbed "Add Shape Tween" menu.
The typical workflow — keyframe, duplicate it (same topology), move
vertices, Add Shape Tween — now morphs between the two. Non-matching
topology falls back to a hold.
Creating the first transform keyframe for a clip instance at frame N left
the curve with a single keyframe, which Hold-extrapolates backward — so
moving it at frame N also moved it on every earlier frame (frame 1).
When SetKeyframeAction creates a brand-new curve for a clip instance and
the clip already existed before `time`, also anchor a keyframe at the
clip's start (its group visibility start, or timeline_start for movie
clips) with the original value. Earlier frames now hold the original
position and the move produces a proper tween from start to N.
Also capture the clip instance's actual on-stage value when keying (its
base transform), instead of a generic 0/identity default, so a new
keyframe doesn't snap the clip to the origin.
Adds VectorLayer::group_visibility_start and tests covering the anchor and
the no-double-anchor case (keying at the clip's own start).
Skyler Lehmkuhl