Both actions were DCEL-stubs (no-ops). They now extract the selected geometry from a
vector layer's active keyframe into a new VectorClip (group vs movie clip) and place a
ClipInstance in its place (identity transform → renders where the geometry was), which
the existing transform-animation system can motion-tween.
- Shared `clip_from_geometry::extract_geometry_to_clip` (+ undo) does the work; the
actions are thin wrappers. Undo snapshots the source graph + removes the clip/instance.
- `extract_subgraph` now DERIVES shared-fill boundary edges internally (an inside edge
still used by a non-extracted fill must be duplicated, not moved) and unions them with
the caller's `explicit_boundary` — so a plain geometry selection needs no boundary
analysis (the selection already includes fill boundary edges via `select_fill`). The
region-select caller keeps passing its lasso boundary.
- Handlers: Group / Convert to Movie Clip on a geometry selection now build these actions
from `selected_fills`/`selected_edges`.
Next: shape tweens (same-topology lerp).
Skyler Lehmkuhl