* 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.
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