Replaces the DCEL "not yet supported" stubs so importing/dropping an image actually
puts it in the vector scene.
- AddShapeAction gains an `image_fill` + `AddShapeAction::image_rect(...)` constructor:
a borderless rectangle (invisible edges) whose enclosed region is paint-bucketed and
tagged with an image asset id. The renderer already prioritises `image_fill`.
- Direct import (auto_place_asset): an imported image is placed centered on the canvas
at native size on a vector layer.
- Drag from the asset library onto the stage: image-filled rect at the drop point
(centered), native size, using the asset's dimensions.
Next: SetImageFillAction + an Info-Panel image-fill picker for existing shapes; then
3.5b container persistence.
Skyler Lehmkuhl