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Textbooks with decorative alphabet strips along page edges produce OCR artifacts (scattered colored letters at x<150 while real content starts at x>=179). Step 4e detects a significant x-gap (>30px) between a small cluster (<15% of total word_boxes) near the page edge and the main content, then removes the border-strip word_boxes. Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>