Add title validation with dcterms:title support

- Add validate_title() method in SupportingTextValidator for exact title
  matching after normalization (case, whitespace, punctuation, Greek letters)
- Add _find_title_fields() in plugin to discover slots implementing
  dcterms:title or having slot_uri dcterms:title
- Add _validate_title() in plugin to yield validation results
- Update _validate_instance() to validate titles from title fields,
  either combined with excerpt or standalone
- Add comprehensive test suite (16 tests) for title validation

Title validation uses exact matching (not substring like excerpt) but
allows for minor orthographic variation including:
- Case insensitivity
- Whitespace normalization
- Punctuation differences
- Greek letter equivalence (α ↔ alpha)

🤖 Generated with [Claude Code](https://claude.com/claude-code)

Co-Authored-By: Claude Opus 4.5 <[email protected]>

Author: cmungall