Surface <!ATTLIST> declarations from the parser and validate attributes against
them. Reimplemented from the DTD attribute-list spec; no upstream code copied.
- DocumentType gains attributeLists (raw ATTLIST bodies per element); the
DoctypeScanner captures them and the EventReader plumbs them through.
- AttributeListParser parses a body into AttributeDeclaration values
(AttributeType: cdata or enumeration; AttributeDefault: required/implied/
fixed/value).
- DTDSchema now checks #REQUIRED presence, #FIXED value match (when present),
and enumerated-value membership, in addition to content models.
- Tokenized types (ID/IDREF/NMTOKEN) parse but are not yet value-enforced.
- 5 attribute tests; 75 total.
