`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.
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