Cap gap_threshold at 25% of zone_w for column detection

In small zones (boxes), intra-phrase gaps inflate the median gap,
causing gap_threshold to become too large to detect real column
boundaries. Cap at 25% of zone width to prevent this.

Example: Box "Pounds and euros" has 4 columns at x≈148,534,751,1137
but gap_threshold was 531 (larger than the column gaps themselves).

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

klausur-service/backend/grid_editor_helpers.py

@@ -282,6 +282,12 @@ def _cluster_columns_by_alignment(
         median_h = sorted(heights)[len(heights) // 2] if heights else 25
         # Column boundary: gap > 3× median gap or > 1.5× median word height
         gap_threshold = max(median_gap * 3, median_h * 1.5, 30)
+        # Cap at 25% of zone width — prevents over-merging in small zones (boxes)
+        # where intra-phrase gaps can inflate the median
+        max_gap = zone_w * 0.25
+        if gap_threshold > max_gap > 30:
+            logger.info("alignment columns: capping gap_threshold %.0f → %.0f (25%% of zone_w=%d)", gap_threshold, max_gap, zone_w)
+            gap_threshold = max_gap
     else:
         gap_threshold = 50