Gets the role-mapping chain: the element's own type in its namespace, then each type the root's RoleMap (default
namespace) or a namespace's RoleMapNS maps it to, until no map applies or a type repeats.
public IReadOnlyList<PdfStructureType> RoleMapping { get; }
ISO 32000-2 §14.7.3 (cycles are permitted, NOTE 2), §14.7.4.2, §14.8.6.2. Empty when the element has no type.