CVE-2026-54897: Oj: Use-After-Free in Oj::Doc Iterators via Reentrant Close

Oj::Doc iterators (each_value, each_child, each_leaf) are vulnerable to a heap use-after-free. When a Ruby block yielded during iteration calls doc.close or d.close, the document’s heap memory is freed while the C iterator is still running. When control returns from the block, the iterator reads from the freed region, producing a use-after-free accessible from pure Ruby.