Introduce --ocr-engine option to select between OCR engines:
- 'auto' (default): Uses Tesseract
- 'tesseract': Explicit Tesseract selection
- 'none': Skip OCR entirely (for PDF processing only)
Key changes:
- Extend OcrEngine ABC with generate_ocr() and supports_generate_ocr()
for direct OcrElement tree output (bypasses hOCR)
- Add get_ocr_engine(options) hook parameter for engine selection
- Implement NullOcrEngine for --ocr-engine none
- Export OcrElement, OcrClass, BoundingBox from ocrmypdf package
- Add ocr_tree support to grafting pipeline
This prepares the foundation for pluggable OCR engines while maintaining
full backward compatibility with existing Tesseract-based workflows.
