fix: apply column union when total_cols >= max (not just >)

Zone 4 found 4 columns incl. page_ref, union also yields 4.
The strict > check prevented union from applying to Zone 0.
Changed to >= so all content zones get the merged column set.

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

klausur-service/backend/grid_editor_api.py

@@ -568,9 +568,11 @@ async def build_grid(session_id: str):
                             for zg in content_zones
                         )
 
-                        # Only apply union if it found more columns than
-                        # any single zone (union adds information)
-                        if total_cols > max_zone_cols:
+                        # Apply union whenever it has at least as many
+                        # columns as the best single zone.  Even with the
+                        # same count the union boundaries are better because
+                        # they incorporate evidence from all zones.
+                        if total_cols >= max_zone_cols:
                             cx_min = min(w["left"] for w in all_words)
                             cx_max = max(
                                 w["left"] + w["width"] for w in all_words