Add Schema.Document compiling an xs:schema into global element declarations
and a named-type table, and validate(_:) checking instance documents end to
end. The parser supports global and local element declarations, named and
inline simple and complex types, sequence/choice/all groups with occurrence,
attribute uses, simple content (restriction/extension), the full facet set,
and recursive types via named references resolved during validation. Extends
ComplexType with a typeReference case and ComplexValidator with a type table
so recursive schemas are representable.
Closes#10
Add ComplexValidator validating an element against a ComplexType: attribute
uses (required, typed, unknown-attribute rejection) and the content model
(empty, simpleContent, elementOnly, mixed). Particles compile to a Thompson
NFA over element names honoring minOccurs/maxOccurs with sequence and choice
compositors; all groups are validated order-independently by counting. Child
elements are validated recursively against their declared types.
Closes#9
