Gets every element reachable from the root, in document order: depth first, each element before its children, children in K order; an element reachable twice is listed once.
public IReadOnlyList<PdfStructureElement> Elements { get; }
ISO 32000-2 §14.7.2, §14.8.2.5 (logical order). A snapshot, built on first use and again after the tree changes.