XSD 1.0 Schema Component Constraint "xsi: Not Allowed" (§3.2.6): an attribute
declaration's {target namespace} must not be the XSI namespace. A schema may
target XSI and declare unqualified attributes (they land in no namespace),
but a top-level attribute, or a local one made qualified by form="qualified"
or attributeFormDefault="qualified", lands in XSI and is forbidden.
Add xsiNamespaceAttributeErrors, scoped to schemas whose target namespace is
XSI, registered as the pre-compile rule attributeDeclarationsNotInXSI. It
rejects corpus attKb018a (qualified attribute-group attributes) and attKa015
(top-level attributes), while attKb018 and attKc018 (local unqualified) stay
valid.
XSTS invalid-schemas-accepted 39 -> 37; valid-schemas-rejected held at 0; no
other bucket moved.