A strict or lax wildcard must resolve elements declared in schema documents an
instance points at through xsi:schemaLocation, which the main schema need not
import. validate(_:schemaLoader:) now reads the instance's xsi:schemaLocation
and xsi:noNamespaceSchemaLocation hints, loads and compiles each referenced
document through the loader, and merges its global declarations in (the primary
schema wins on conflict). The plain validate(_:) is unchanged; the XSTS runner
resolves hints relative to the instance file.
XSTS: valid instances rejected 308 to 288 (the particle clusters whose
wildcard-matched elements live in a separate, instance-referenced schema).
Baseline updated; regression tests cover cross-document resolution, the loaded
type being enforced, and strict failure without the loader.
Refs #147, #129.
