proposal: add-layout-preprocessing for improved table detection

Problem: PP-Structure misses tables with faint lines/borders
Solution: Preprocess images (contrast, sharpen) for layout detection
- Preprocessed image only used for layout detection
- Original image preserved for element extraction (quality)

Includes: proposal.md, design.md, tasks.md, spec delta

🤖 Generated with [Claude Code](https://claude.com/claude-code)

Co-Authored-By: Claude <noreply@anthropic.com>

openspec/changes/add-layout-preprocessing/specs/ocr-processing/spec.md

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+## ADDED Requirements
+
+### Requirement: Layout Detection Image Preprocessing
+
+The system SHALL provide optional image preprocessing to enhance layout detection accuracy for documents with faint lines, low contrast, or poor scan quality.
+
+#### Scenario: Preprocessing improves table detection
+- **GIVEN** a document with faint table borders that PP-Structure fails to detect
+- **WHEN** layout preprocessing is enabled
+- **THEN** the system SHALL preprocess the image before layout detection
+- **AND** contrast enhancement SHALL make faint lines more visible
+- **AND** PP-Structure SHALL receive the preprocessed image for layout detection
+
+#### Scenario: Image element extraction uses original quality
+- **GIVEN** an image element detected by PP-Structure from preprocessed input
+- **WHEN** the system extracts the image element
+- **THEN** the system SHALL crop from the ORIGINAL image, not the preprocessed version
+- **AND** the extracted image SHALL maintain original quality and colors
+
+#### Scenario: Preprocessing can be disabled
+- **GIVEN** `layout_preprocessing_enabled` is set to false in configuration
+- **WHEN** OCR track processing runs
+- **THEN** the system SHALL skip preprocessing
+- **AND** PP-Structure SHALL receive the original image directly
+
+#### Scenario: CLAHE contrast enhancement
+- **WHEN** `layout_preprocessing_contrast` is set to "clahe"
+- **THEN** the system SHALL apply Contrast Limited Adaptive Histogram Equalization
+- **AND** the enhancement SHALL not over-saturate already bright regions
+
+#### Scenario: Sharpening enhances faint lines
+- **WHEN** `layout_preprocessing_sharpen` is enabled
+- **THEN** the system SHALL apply unsharp masking to enhance edges
+- **AND** faint table borders SHALL become more detectable
+
+#### Scenario: Optional binarization for extreme cases
+- **WHEN** `layout_preprocessing_binarize` is enabled
+- **THEN** the system SHALL apply adaptive thresholding
+- **AND** this SHALL be used only for documents with very poor contrast
+
+### Requirement: Preprocessing Track Isolation
+
+The layout preprocessing feature SHALL only affect layout detection input without impacting other processing components.
+
+#### Scenario: Raw OCR is unaffected
+- **GIVEN** layout preprocessing is enabled
+- **WHEN** Raw OCR processing runs
+- **THEN** Raw OCR SHALL use the original image
+- **AND** text detection quality SHALL not be affected by preprocessing
+
+#### Scenario: Preprocessed image is temporary
+- **GIVEN** an image is preprocessed for layout detection
+- **WHEN** layout detection completes
+- **THEN** the preprocessed image SHALL NOT be persisted to storage
+- **AND** only the original image and element crops SHALL be saved