A <data type=> pattern now carries a SimpleType (base + facets) rather
than a bare built-in, and its <param name=>value</param> children
populate the facets (minLength, maxLength, length, pattern, min/max
inclusive/exclusive, total/fraction digits), reusing the XSD datatype
facet validation. So a constrained datatype like xsd:string with
minLength rejects too-short values.
datatypeLibrary resolution, typed <value> comparison, and the RNC
datatype parameter blocks remain.
Part of #79
