The border strip filter (Step 4e) used the LARGEST x-gap which incorrectly
removed base words along with edge artifacts. Now uses a two-stage approach:
1. _filter_border_strip_words() pre-filters raw words BEFORE column detection,
scanning from the page edge inward to find the FIRST significant gap (>30px)
2. Step 4e runs as fallback only when pre-filter didn't apply
Session 4233 now correctly detects 3 columns (base word | oder | synonyms)
instead of 2. Threshold raised from 15% to 20% to handle pages with many
edge artifacts. All 4 ground-truth sessions pass regression.
Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>
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