xs:ID, xs:IDREF, and xs:IDREFS were validated only lexically (as NCNames); the
document-wide rules that every ID is unique and every IDREF/IDREFS item matches
some ID were not enforced, so duplicate IDs and dangling references passed. The
tree validator now records ID and reference values during its typed walk over
elements and attributes (a new IDTracker), and reports duplicate IDs and
unresolved references after the whole document is seen, so a forward reference
resolves against an ID declared later.
XSTS: invalid instances accepted 179 to 166. Baseline updated; regression tests
cover unique IDs, a forward reference, a duplicate ID, an unresolved IDREF, and
IDREFS item resolution. The ID/IDREF helpers live in a ComplexValidator
extension to keep the type within the file/body length limits.
Refs #147, #129.
