libxml-rs 0.1.0-alpha.7

Phase 6: Validation family (DTD, XML Schema, RELAX NG, Schematron). Native-Rust forensic reimplementation of libxml2+libxslt with C ABI drop-in replacement. 802 tests passing, DTD validation, XML Schema parser+validator (45+ built-in datatypes), RELAX NG grammar+validation, ISO Schematron rules+patterns, full C ABI exports.