Resolves, measures, lays out and paints. Once.
Synchronous, because the addon's is: the painted surface holds Skia types
that are not Send, so the paint cannot leave the loop the way render's
does. Root is asynchronous anyway — a page builder may fetch.
The native side of
Root.Declared rather than imported, for the reason NativeCanvas is: this file compiles without the addon, and the shape the addon has to satisfy is written down in one place.