The streaming XSD validator ignored an element's default/fixed value constraint
in simple-content validation, so an empty element with a value constraint was
validated against the empty string (a false positive), and a present fixed
value was compared as a raw string rather than in the type's value space.
Mirror the tree path's validateChild for the simple-typed element:
- an empty element takes its constraint value via a shared effectiveSimpleText
helper both paths use; the constraint comes from the matched particle (keyed
by namespaced identity), not the flat by-local-name elementConstraints map,
so same-local-name elements in different types do not collide.
- a present fixed value is compared in the type's value space and against the
matched particle's fixed value (elementFixedErrors with valueType and
particleFixed), not a raw string compare ("05" equals int fixed "5").
The particle constraint is looked up from the parent's content model at
element-close time. Streaming now agrees with the tree-path oracle on value
constraints, verified by an adversarial differential (13 configurations).
No XSTS bucket changes; gate held at 0/37/0/26.