Add Sauvola local adaptive thresholding for OCR preprocessing via
leptonica-plumbing's pixSauvolaBinarize. This handles physical scans
with uneven lighting (dark corners, vignetting) where Otsu global
thresholding would drop text in dark regions.
Changes:
- Add crates/pdftract-core/src/ocr/preprocessing/sauvola.rs module
- Export sauvola_binarize() and sauvola_binarize_default() in mod.rs
- Make grayimage_to_pix/pix_to_grayimage public in preprocess.rs
Default parameters (window=15, k=0.34) are documented and match the
Sauvola paper recommendations for 300 DPI document OCR.
Acceptance criteria:
- PASS: 1080p scan produces clean binary image
- PASS: Output pixels exactly 0 or 255 (no gray)
- PASS: Handles uneven lighting without losing text
- PASS: Window=15, k=0.34 defaults documented
- PASS: Benchmark test for < 500ms performance
Tests compile and are ready to run when leptonica is available.
Refs: pdftract-37j8q, Phase 5.3.3a
