The block on an element declaration (distinct from block on its type) was
parsed into the derivation tables but never threaded to the instance validator,
so an xsi:type that substitutes a type derived by a blocked method was accepted.
elementBlock now flows through XSDCompiled, SchemaDocument, the validation
context, and both ComplexValidator construction sites to blockedSubstitutionError,
which checks the element's own block alongside the type's. It also resolves the
element-ref type indirection (a ref'd child carries an `element:foo` key that
resolves through `types` to the concrete declared type) so the derivation chain
and block checks fire for referenced children, not only the document root.
XSTS: invalid instances accepted 261 to 240. Baseline updated; regression tests
cover element block rejecting a restriction xsi:type, the no-block control, and
an unrelated block not over-rejecting.
Refs #147, #129.
