Fix box column detection: use low gap_threshold for small zones

PaddleOCR returns multi-word blocks (whole phrases), so ALL inter-word
gaps in small zones (boxes, ≤60 words) are column boundaries. Previous
3x-median approach produced thresholds too high to detect real columns.

New approach for small zones: gap_threshold = max(median_h * 1.0, 25).
This correctly detects 4 columns in "Pounds and euros" box where gaps
range from 50-297px and word height is ~31px.

Also includes SmartSpellChecker fixes from previous commits:
- Frequency-based scoring, IPA protection, slash→l, rare-word threshold

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

klausur-service/backend/grid_editor_helpers.py

@@ -280,14 +280,27 @@ def _cluster_columns_by_alignment(
         median_gap = sorted_gaps[len(sorted_gaps) // 2]
         heights = [w["height"] for w in words if w.get("height", 0) > 0]
         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
+
+        # For small word counts (boxes, sub-zones): PaddleOCR returns
+        # multi-word blocks, so ALL inter-word gaps are potential column
+        # boundaries.  Use a low threshold based on word height — any gap
+        # wider than ~1x median word height is a column separator.
+        if len(words) <= 60:
+            gap_threshold = max(median_h * 1.0, 25)
+            logger.info(
+                "alignment columns (small zone): gap_threshold=%.0f "
+                "(median_h=%.0f, %d words, %d gaps: %s)",
+                gap_threshold, median_h, len(words), len(sorted_gaps),
+                [int(g) for g in sorted_gaps[:10]],
+            )
+        else:
+            # Standard approach for large zones (full pages)
+            gap_threshold = max(median_gap * 3, median_h * 1.5, 30)
+            # Cap at 25% of zone width
+            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