Render against a document other than window.document #3056

Moved from ryansolid/dom-expressions#253 / the layering discussion on solid#1740.

template() / createElement always hit global document. Scoped custom element registries and shadow-root-backed documents need the mount root’s document-like object instead.

#1740 put a document field on the reactive owner. That was rejected because it puts DOM-specific state into the owner. The gap is still real; the enhancement is a renderer-owned document (or equivalent) that does not leak into rxcore.