feat: box-aware column detection — exclude box content from global columns

- Enrich column geometries with original full-page words (box-filtered)
  so _detect_sub_columns() finds narrow sub-columns across box boundaries
- Add inline marker guard: bullet points (1., 2., •) are not split into
  sub-columns (minimum gap check: 1.2× word height or 20px)
- Add box_rects parameter to build_grid_from_words() — words inside boxes
  are excluded from X-gap column clustering
- Pass box rects from zones to words_first grid builder
- Add 9 tests for box-aware column detection

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

klausur-service/backend/ocr_pipeline_api.py

@@ -2543,7 +2543,15 @@ async def detect_words(
                     })
                 wf_word_dicts = abs_words
 
-        cells, columns_meta = build_grid_from_words(wf_word_dicts, img_w, img_h)
+        # Extract box rects for box-aware column clustering
+        box_rects = []
+        for zone in zones:
+            if zone.get("zone_type") == "box" and zone.get("box"):
+                box_rects.append(zone["box"])
+
+        cells, columns_meta = build_grid_from_words(
+            wf_word_dicts, img_w, img_h, box_rects=box_rects or None,
+        )
         duration = time.time() - t0
 
         # Apply IPA phonetic fixes