Surface the parsed <!DOCTYPE> internal subset's <!ELEMENT> declarations from the
parser and validate a tree against them. Reimplemented from the DTD content-model
spec; no upstream code copied.
- DoctypeScanner captures <!ELEMENT name model> declarations; the parser surfaces
them as Parsing.DocumentType (via parseWithDocumentType).
- Validation.ContentModel + ContentModelParser parse a model string (EMPTY, ANY,
(#PCDATA), mixed, and element content with sequence/choice and ?/*/+).
- ContentModelMatcher matches the child element-name sequence against a model as
a regular language (NFA-position matching).
- Validation.DTDSchema.validate reports issues for EMPTY/PCDATA/mixed violations,
element content with stray text, and child sequences that do not match.
- PureXML.validateAgainstInternalDTD(_:) parses and validates in one call.
- ATTLIST validation is out of scope. 9 tests; 70 total.
