A complex type derived by restriction is checked at schema compile to accept
a structural subset of its base, closing the declared XSD 1.0 spec gap.
ParticleRestriction covers element name matching, element-vs-wildcard
admission, wildcard narrowing, occurrence-range subsumption, and the group
recursions: sequence/all order-preserving with emptiable skips (Recurse),
choice mapping (RecurseLax), sequence-into-choice (MapAndSum, approximated by
per-particle checks, a documented simplification), sequence-into-all
(RecurseUnordered), and RecurseAsIfGroup for a lone element against a group.
An unfaithful restriction throws SchemaError.invalidRestriction(type:base:
reason:). All 834 pre-existing tests pass unchanged (no false positives on
the existing restriction-using schemas); 7 new tests cover the accept and
reject paths plus a conformance-harness pair.
Closes#117
