Add optional smart routing utilities for consumers:
- pre_analyze(): lightweight file classification (pdf_text/pdf_scanned/image/office/html)
- quality_ok(): quality assessment for EngineResult (text length, confidence, gibberish ratio)
- QualityThresholds: configurable via dataclass or env vars
- Tesseract engine: populate confidence field and declare capabilities
- 50 new tests (225 total)