- Add table confidence scoring algorithm (0.0-1.0 scale)
- Implement adaptive column detection using character-width-based sizing
- Add plain text fallback for low-confidence tables (<0.5)
- Expose table quality metrics via StructuralHints:
  - TablesDetected, LowConfidenceTables, MinTableConfidence
- Add Known Limitations section to README
- Fix garbled markdown output from complex PDF tables

Closes: PDF table extraction quality issue