The substitution filter dropped only members whose type derived from the head's
type by a blocked method (extension/restriction). The `substitution` keyword in
a head's block forbids substitution-group substitution outright, whatever the
member's type, and was ignored, so a same-type member still substituted for a
head declared block="substitution". Members are now dropped entirely when the
head blocks substitution.
XSTS: invalid instances accepted 238 to 231, valid schemas rejected 75 to 74.
Baseline updated; regression tests cover the substitution block rejecting a
member, plain heads still validating, and the no-block control.
Refs #147, #129.
Mihaela Mihaljevic
47e20ff90b