The renderer registry, generic over whatever a binding calls a renderer.
The core never renders, so it only enforces that the registry covers the
vocabulary; a React binding instantiates R with its component type.
A copy. An engine takes one at creation, so registering a renderer
afterwards cannot change what an existing engine renders, which is
what "immutable after creation" has to mean.
The renderer registry, generic over whatever a binding calls a renderer. The core never renders, so it only enforces that the registry covers the vocabulary; a React binding instantiates
Rwith its component type.