Convert the five composition/redefine compile checkers to located findings,
the last of the #169 burndown (#210-#217): a resolved schemaLocation whose
content is not a schema underlines the include/import/redefine reference, a
redefined type that does not restrict or extend itself underlines the
redefined type, a redefined group or attribute group with a malformed
self-reference underlines the offending reference or redefinition, and a
redefined attribute group or model group that does not restrict its original
underlines the redefinition.
To locate the referenced-schema rule, failedSchemaReferences carries the
referencing node, captured at the load-failure site in XSDContext. Only a
top-level reference's node belongs to the main schema tree the diagnostic
resolves against; a failure nested in a loaded sub-document is captured from
a throwaway parse, so it stays unlocated (the prior schema-root behavior)
rather than mis-resolve, a strict improvement.
Every predicate, aggregation order, and reason string is unchanged; only the
location each finding carries differs, verified conformance-neutral by the
full XSTS suite (14383 groups, pinned counts unchanged). The redefine
checkers had exactly one caller each, so each is renamed in place and the
.unlocated wrap dropped. Adds a located-diagnostics test per node-locatable
checker; the attribute-group and model-group restriction redefine checkers
share the same redefinition-node idiom and are exercised by the XSTS suite.