docs: archive enable-doc-orientation-detection proposal

Feature implementation completed and tested successfully.
- PP-StructureV3 orientation detection enabled
- Page dimensions correctly swapped for 90°/270° rotations
- Output PDF now displays landscape content correctly

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

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

openspec/changes/archive/2025-12-11-enable-doc-orientation-detection/proposal.md

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+# Enable Document Orientation Detection
+
+## Summary
+Enable PP-StructureV3's document orientation classification feature to correctly handle PDF scans where the content orientation differs from the PDF page metadata.
+
+## Problem Statement
+Currently, when a portrait-oriented PDF contains landscape-scanned content (or vice versa), the OCR system produces incorrect results because:
+
+1. **pdf2image** extracts images based on PDF metadata (e.g., `Page size: 1242 x 1755`, `Page rot: 0`)
+2. **PP-StructureV3** has `use_doc_orientation_classify=False` (disabled)
+3. The OCR attempts to read sideways text, resulting in poor recognition
+4. The output PDF has wrong page dimensions
+
+### Example Scenario
+- Input: Portrait PDF (1242 x 1755) containing landscape-scanned delivery form
+- Current output: Portrait PDF with unreadable/incorrect text
+- Expected output: Landscape PDF (1755 x 1242) with correctly oriented text
+
+## Proposed Solution
+Enable document orientation detection in PP-StructureV3 and adjust page dimensions based on the detected rotation:
+
+1. **Enable orientation detection**: Set `use_doc_orientation_classify=True` in config
+2. **Capture rotation info**: Extract the detected rotation angle (0°/90°/180°/270°) from PP-StructureV3 results
+3. **Adjust dimensions**: When 90° or 270° rotation is detected, swap width and height for the output PDF
+4. **Use OCR coordinates directly**: PP-StructureV3 returns coordinates based on the rotated image, so no coordinate transformation is needed
+
+## PP-StructureV3 Orientation Detection Details
+According to PaddleOCR documentation:
+- **Stage 1 preprocessing**: `use_doc_orientation_classify` detects and rotates the entire page
+- **Output format**: `doc_preprocessor_res` contains:
+  - `class_ids`: [0-3] corresponding to [0°, 90°, 180°, 270°]
+  - `label_names`: ["0", "90", "180", "270"]
+  - `scores`: confidence scores
+- **Model accuracy**: PP-LCNet_x1_0_doc_ori achieves 99.06% top-1 accuracy
+
+## Scope
+- Backend only (no frontend changes required)
+- Affects OCR track processing
+- Does not affect Direct or Hybrid track
+
+## Risks and Mitigations
+| Risk | Mitigation |
+|------|------------|
+| Model might incorrectly classify mixed-orientation pages | 99.06% accuracy is acceptable; `use_textline_orientation` (already enabled) handles per-line correction |
+| Coordinate mismatch in edge cases | Thorough testing with portrait, landscape, and mixed documents |
+| Performance overhead | Orientation classification adds ~100ms per page (negligible vs total OCR time) |
+
+## Success Criteria
+1. Portrait PDF with landscape content produces landscape output PDF
+2. Landscape PDF with portrait content produces portrait output PDF
+3. Normal orientation documents continue to work correctly
+4. Text recognition accuracy improves for rotated documents