cvc-identity-constraint.3 / c-fields-xpaths require an identity-constraint field to
identify a node with a simple type. The prior fix rejected a field whose selected
instance node carries element children (structural). This adds the case where the
field's DECLARED type is a complex type with non-simple content (empty or
element-only/mixed): an attributes-only element resolves to an empty-content complex
type, so its instance node is childless yet not simple-typed.
The field's target type is resolved through the schema type model (the selector
target's declared type, then the field's same-named child element in its content
model) and classified. A simple or simpleContent target, a type reached only
through a named reference, a complex field path, or an unresolvable target is left
alone, so no valid field is rejected. The resolution helpers live in a separate
extension file to keep the validator within the type-body limit.
This catches W3C XSTS idG006. XSTS invalid-instances-accepted 23 -> 22,
valid-instances-rejected held at 0.
