Resolves all same-document $refs inline so sandbox code never chases references.

Circular refs become %{"$circular" => name}; external or dangling refs become %{"$unresolved" => ref}. Sandbox code can detect both markers.

A ref's expansion is memoized by pointer, but only cached when it completed without producing a $circular marker anywhere inside it — a cycle-hitting expansion is stack-dependent (its shape depends on which refs are currently being expanded above it) and must be recomputed for every call site.