fix: normalize word_box order to reading order for frontend display (Step 5j)

The frontend renders colored cells from the word_boxes array order,
not from cell.text. After post-processing steps (5i bullet removal etc),
word_boxes could remain in their original insertion order instead of
left-to-right reading order. Step 5j now explicitly sorts word_boxes
using _group_words_into_lines before the result is built.

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

klausur-service/backend/grid_editor_api.py

@@ -23,7 +23,7 @@ from fastapi import APIRouter, HTTPException, Request
 from cv_box_detect import detect_boxes, split_page_into_zones
 from cv_vocab_types import PageZone
 from cv_color_detect import detect_word_colors, recover_colored_text
-from cv_ocr_engines import fix_cell_phonetics, fix_ipa_continuation_cell, _text_has_garbled_ipa, _lookup_ipa, _words_to_reading_order_text
+from cv_ocr_engines import fix_cell_phonetics, fix_ipa_continuation_cell, _text_has_garbled_ipa, _lookup_ipa, _words_to_reading_order_text, _group_words_into_lines
 from cv_words_first import _cluster_rows, _build_cells
 from ocr_pipeline_session_store import (
     get_session_db,
@@ -2317,6 +2317,24 @@ async def _build_grid_core(session_id: str, session: dict) -> dict:
                           if (c.get("word_boxes") or c.get("text", "").strip())]
         logger.info("Step 5i: removed %d bullet/artifact word_boxes", bullet_removed)
 
+    # 5j. Normalise word_box order to reading order (group by Y, sort by X).
+    # The frontend renders colored cells from word_boxes array order
+    # (GridTable.tsx), so they MUST be in left-to-right reading order.
+    wb_reordered = 0
+    for z in zones_data:
+        for cell in z.get("cells", []):
+            wbs = cell.get("word_boxes") or []
+            if len(wbs) < 2:
+                continue
+            lines = _group_words_into_lines(wbs, y_tolerance_px=15)
+            sorted_wbs = [w for line in lines for w in line]
+            # Check if order actually changed
+            if [id(w) for w in sorted_wbs] != [id(w) for w in wbs]:
+                cell["word_boxes"] = sorted_wbs
+                wb_reordered += 1
+    if wb_reordered:
+        logger.info("Step 5j: re-ordered word_boxes in %d cells to reading order", wb_reordered)
+
     duration = time.time() - t0
 
     # 6. Build result