Close the last superficially-located checker (#169): referenceFindings was
dispatched as located but its body wrapped the whole aggregate in
.unlocated, so every reference-resolution error still collapsed onto the
schema root. Convert all six sub-collectors to located findings, each
attaching to the node in scope: the core type/base/itemType/memberTypes/ref/
substitutionGroup walk and the xs:-namespace builtin check locate on the
referencing element; the simpleContent base-kind check on the restriction;
the un-imported-namespace check on the referencing element; the
redefine-component-existence check on the redefine's child component; and
the import-namespace-agreement check on the import element.
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). Removes two dead
helpers exposed along the way: the unused referenceErrors [String] wrapper
and the unused locatedFindings helper in SchemaCompileValidation. The outer
aggregation is rewritten from a single + chain to incremental appends to keep
the type-checker within budget. Adds a located-diagnostics test per
node-locatable sub-collector; the loader-dependent redefine/import families
are exercised by the XSTS suite.
