src-import.3.1/3.2: an xs:import's namespace attribute must equal the
targetNamespace of the schema document it resolves to (both present and equal,
or both absent). Importing a no-namespace document under a declared namespace,
or one whose target namespace differs, is now rejected.
importNamespaceErrors (new SchemaImportNamespace.swift) builds a resolved
schemaLocation -> targetNamespace map from the loaded containers and compares
each import node's namespace to it. Checked only when the schemaLocation
resolved to a loaded schema (an unresolved import is the processor's choice,
not an error) -- the false-positive boundary; imports keep their own target
namespace so no chameleon adjustment applies.
XSTS: invalid-schemas-accepted 87 to 84 (schF6/schG10/schG13),
valid-schemas-rejected held at 0, no other bucket moved. Differential:
agree-valid unchanged at 3161, PureXML-stricter unchanged at 87 over 9618
schemas. Second step of the cross-document composition campaign (#161).