Column_1 data showed avg_len=1.0 with 13 single-char cells (alphabet
letters from sidebar). Old fill_ratio check (76% > 35%) missed it.
New criteria: avg_len ≤ 1.5 AND ≥ 70% single chars → removes column.
Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>