Every QName a schema names must resolve to a declared component or a built-in:
type/base/itemType/memberTypes to a type, element/attribute/group/attributeGroup
ref to the matching global component, and element substitutionGroup to a global
element. An undeclared reference was previously accepted at compile time and only
surfaced lazily during instance validation, if at all.
References are matched by local name, with the value trimmed (a QName attribute
is whiteSpace=collapse). The whole check is skipped when the document declares an
import/include/redefine, since the default compile does not load externals and
the pools would be incomplete; that guard is gated on the XSD namespace so a
foreign element named "import" does not trip it.
XSTS: invalid schemas accepted 1092 to 1051 (-41); valid schemas rejected held
at 72 (an interim version regressed one valid schema on a whitespace-padded
base=" xs:string ", fixed by trimming); instance buckets unchanged.
