Extend an identity error's coding path past the target element to the single
field the constraint names: an attribute (list/item[2]/@id) or a child element
(.../code), resolved from the field XPath. Multi-field or complex-path
constraints stay on the target. Also add a cross-scope keyref test confirming
a keyref resolves against a key declared on an ancestor element. Updates the
identity-path tests to the more precise locations.
Closes#104
Mihaela Mihaljevic
963c7c74eb