Rux runtime, milestones M2–M9.
The document model: loads a .rux file, resolves its use component imports
(loading each imported .rux), builds the script [Engine] (merging the main
and component scripts, registering host functions), and builds the renderable
tree with bindings, directives, and component expansions resolved. Running an
@tap handler mutates engine state; rebuild refreshes the tree.