Add Phase 4.7 BrokenVector escalation: when a page classified as Vector
has readability score < 0.5, escalate to BrokenVector and route to Phase 5.5 OCR.
Changes:
- Add PageClass::can_escalate_to_broken_vector() method
- Add apply_broken_vector_escalation() function with cfg(ocr) gating
- Add 13 comprehensive tests covering all escalation scenarios
Closes: pdftract-5v1l9
