Embedder API gaps for implements/external-id #14862

A few gaps in how implements/external-id are exposed through the embedder API:

• Component::get_export and Instance::get_export return only the ComponentItem, so an export's attributes are only reachable by iterating component_type().exports(). The data is stored in info::Export::Instance but never read.
• Root-level type imports (e.g. (import "t" (external-id "x") (type (eq $u)))) are missing from component_type().imports(), while root type exports are present.
• ComponentExtern's lifetime is tied to the &Engine borrow even though its strings borrow from the type (ty.get_import(&engine.clone(), "p") doesn't compile), and the struct isn't #[non_exhaustive], so adding a field (say, a version suffix for canonical names) would be a breaking change.