…attrs

Extends strict XSD validation with several fixes that eliminate false
positives on real-world NAS files (91k+ errors on WFS Transaction files
reduced to 0):

- Merge xmlns prefix declarations from included schemas into root
  prefix_map so QName resolution (e.g. gmd:LI_Lineage) works when the
  prefix is declared in an include, not the root schema document
- Store prefix_map on ImportedSchema for cross-namespace resolution
- Inherit attributes from simpleContent extension chains via
  resolve_simple_content_base_attributes (fixes gml:MeasureType uom,
  gml:CodeWithAuthorityType codeSpace)
- Handle <restriction> branch in collect_simple_content_attributes
  (not just <extension>)
- Search imported schemas in resolve_element_ref for unprefixed refs
- Scan imported types by local name when prefix is unknown
- Treat elements with no type_ref/inline_type/element_ref as anyType
  (valid per XSD spec) instead of reporting unresolved-type errors
- Resolve element namespace via imported prefix_maps before
  element-based fallback
- Inline named model groups via two-pass parsing and group_defs map
- Resolve substitution group members to their own type declarations
  (not abstract head) for correct content model validation
- Loop on unbounded Group particles in sequence validation
- report_unexpected parameter avoids double-reporting in group context