ValidatePdfUa2StructureElements function

ValidatePdfUa2StructureElements runs every focused PDF/UA-2 rule family over the flattened structure data and unions the resulting issue sets

The aggregated families are ValidatePdfUa2NamespaceSemantics, ValidatePdfUa2Containment, ValidatePdfUa2TableGrid, ValidatePdfUa2AnnotationSemantics, ValidatePdfUa2FormulaSemantics, ValidatePdfUa2AssociatedFileSemantics, and ValidatePdfUa2DocumentSemantics

Like each focused function it is a pure data routine that needs no pdfium.dll, so it can be unit-tested with synthetic element arrays

• ValidatePdfUaStructureElements merges this union into its result when Conformance = puc2, which is the path TPdf.ValidatePdfUa uses
• PdfUa2StructureHasIssue wraps this function for a single-issue query
• Do not confuse it with ValidatePdfUaStructureElements, the PDF/UA-1 oriented validator that takes a conformance parameter