* feat(scene-graph)!: make a stroke a paint
Stroke extends Fill, so a stroke carries the same paint vocabulary a
fill does instead of a lone color. Every construction site now states
a solid paint type, which keeps today's behaviour exactly; rendering,
.fig conversion, and the stroke panel still read solid strokes only.
copyFill is generic over the paint shape so copyStroke reuses it
rather than repeating the deep copy of gradient stops, transforms and
pattern fields.
Groundwork for the gradient and image strokes in #797.
* test(scene-graph): cover a stroke's nested paint data in copyStroke
A stroke is a paint now, so its gradient stops and transform must copy
as deeply as a fill's; the fixture was solid and proved only the color
and dash pattern.
Scene Graph unit tests move to packages/scene-graph/tests with a package-local assert helper and test type-checking; five Core- and fig-owned tests move to their owners' engine homes. check:test-homes rejects any new test under tests/engine against a reviewed baseline so the migration debt only shrinks.
- Add mixed-selection cap, join, and miter-limit controls with one-step undo
- Apply node miter limits across CanvasKit stroke paths and vector outline caches
- Preserve imported and edited stroke geometry through Plugin API and .fig roundtrips
- Move newly split tests under explicit domain subfolders
- Update nested helper imports after the moves
- Add a lint guard against repeating existing sibling domains as filename prefixes
