diag: add row-clustering logging for small/box zones

Logs word positions, median height, Y tolerance, and resulting
rows for zones with <= 30 words to diagnose row merging issues.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

klausur-service/backend/grid_editor_api.py

@@ -365,6 +365,29 @@ def _build_zone_grid(
 
     # Cluster rows first (needed for column alignment analysis)
     rows = _cluster_rows(zone_words)
+
+    # Diagnostic logging for small zones (box zones typically)
+    if len(zone_words) <= 30:
+        import statistics as _st
+        _heights = [w['height'] for w in zone_words if w.get('height', 0) > 0]
+        _med_h = _st.median(_heights) if _heights else 20
+        _y_tol = max(_med_h * 0.5, 5)
+        logger.info(
+            "zone %d row-clustering: %d words, median_h=%.0f, y_tol=%.1f → %d rows",
+            zone_index, len(zone_words), _med_h, _y_tol, len(rows),
+        )
+        for w in sorted(zone_words, key=lambda ww: (ww['top'], ww['left'])):
+            logger.info(
+                "  zone %d word: y=%d x=%d h=%d w=%d '%s'",
+                zone_index, w['top'], w['left'], w['height'], w['width'],
+                w.get('text', '')[:40],
+            )
+        for r in rows:
+            logger.info(
+                "  zone %d row %d: y_min=%d y_max=%d y_center=%.0f",
+                zone_index, r['index'], r['y_min'], r['y_max'], r['y_center'],
+            )
+
     # Use global columns if provided, otherwise detect per zone
     columns = global_columns if global_columns else _cluster_columns_by_alignment(zone_words, zone_w, rows)