Per bead pdftract-1ob acceptance criteria:
- Add page_type_string function to page_class.rs that implements the
stable mapping from (PageClass, ocr_succeeded, has_text, has_images)
to page_type JSON enum values per Phase 5.1.1 spec
- Add PageClass impl with as_type_str() and can_escalate_to_broken_vector()
methods
- Re-export PageClassification and page_type_string from lib.rs
- Add comprehensive unit tests:
* test_page_type_string_*: tests for each PageClass variant and override cases
* test_page_type_string_exhaustive_combinations: validates all 32 combinations
* test_page_type_enum_schema_set: verifies output equals the 6 schema values
* test_page_class_as_type_str: tests as_type_str method
* test_page_class_can_escalate_to_broken_vector: tests escalation eligibility
Closes: pdftract-1ob
d0ea4a7085