Composition / API
foldkit-composition/richtext
foldkit-composition/richtext: rich text held as a Block's prop.
A page and a rich-text document are two documents on purpose, and they meet
in one direction: a Block holds a foldkit-richtext Document as its body.
The Block names the Kit the body accepts, so validating the page validates
the body too, each finding reported at its path as composition:nested.