Character references now require lowercase 'x' for hex and a referenced code
point inside the XML Char production (rejecting �, , ,
surrogates, and X). The reserved 'xml' processing-instruction target
(any case) is legal only as the declaration at the document's very first
bytes: whitespace or a comment before <?xml ...?>, a declaration inside
content or after the root, and case variants like <?XML?> are all rejected.
Comments and PI data are checked for the XML Char production like content.
not-wf/sa: 109/186 rejected (from 93); valid/sa unchanged at 118/119; all
846 suite tests green, baseline rebased to the exact 77 remaining (the
internal-subset grammar and entity replacement-text classes on #120).
Part of #120, #105