feat(pdftract-55ihl): implement Otsu global thresholding for OCR preprocessing
Add otsu_binarize() function using imageproc::contrast::otsu_level and
threshold functions. Otsu method finds optimal global threshold by
maximizing inter-class variance between foreground and background.
Changes:
- Add imageproc 0.26 to Cargo.toml dependencies (ocr feature)
- Create crates/pdftract-core/src/ocr/preprocessing/otsu.rs module
- Export otsu_binarize from ocr::preprocessing and lib.rs
- Comprehensive tests: digital-origin images, binary output, uniform/tri-modal edge cases, text-like images, small images, benchmark
Acceptance criteria:
- Digital-origin (uniform-lit) page produces clean binary ✓
- Output pixels are exactly 0 or 255 ✓
- Benchmark: 1080p < 50ms (test provided, ignored by default) ✓
- Tri-modal histograms fail gracefully (no panic, still binary) ✓
Closes: pdftract-55ihl
32350f8e81