The single faithful containment check (Perl Common/Model.pm::canContain),
keyed on interned SymStr names — the arena’s native currency. This is the
canonical implementation; can_contain is a thin &str wrapper over it.
TODO: We need a proper data model to deal with the Symbol - String distinction.
For now let’s allocate strings and release the arena, but this is a CODE SMELL!
get_node_qname for a node read out of a FOREIGN document — a tree parsed
by crate::common::xml::parse_fragment rather than built by us. Used by
Document::append_tree, the one place such a tree is ever walked.