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/tests/test_grid_editor_api.py

@@ -1046,3 +1046,50 @@ class TestBlueBulletFilter:
         # so it doesn't matter which one is removed — the text stays correct.
         # The key thing is ONE of the duplicates is removed.
         assert True  # Removing either duplicate is correct
+
+
+# ---------------------------------------------------------------------------
+# Word_box reading order normalisation (Step 5j)
+# ---------------------------------------------------------------------------
+
+class TestWordBoxReadingOrder:
+    """Verify word_boxes are sorted into reading order for frontend rendering."""
+
+    def test_single_line_sorted_by_left(self):
+        """Words on same Y line sorted by X (left) position."""
+        from cv_ocr_engines import _group_words_into_lines
+        wbs = [
+            {"text": "up",        "left": 376, "top": 264, "width": 22, "height": 19},
+            {"text": "tie",       "left": 284, "top": 264, "width": 23, "height": 14},
+            {"text": "sb/sth",    "left": 309, "top": 264, "width": 57, "height": 20},
+        ]
+        lines = _group_words_into_lines(wbs, y_tolerance_px=15)
+        sorted_wbs = [w for line in lines for w in line]
+        assert [w["text"] for w in sorted_wbs] == ["tie", "sb/sth", "up"]
+
+    def test_two_lines_preserves_line_order(self):
+        """Words on two Y lines: first line first, then second line."""
+        from cv_ocr_engines import _group_words_into_lines
+        wbs = [
+            {"text": "b)", "left": 100, "top": 290, "width": 20, "height": 15},
+            {"text": "cat", "left": 50, "top": 264, "width": 30, "height": 15},
+            {"text": "dog", "left": 100, "top": 264, "width": 30, "height": 15},
+            {"text": "a)", "left": 50, "top": 290, "width": 20, "height": 15},
+        ]
+        lines = _group_words_into_lines(wbs, y_tolerance_px=10)
+        sorted_wbs = [w for line in lines for w in line]
+        assert [w["text"] for w in sorted_wbs] == ["cat", "dog", "a)", "b)"]
+
+    def test_already_sorted_unchanged(self):
+        """Already-sorted word_boxes stay in same order."""
+        from cv_ocr_engines import _group_words_into_lines
+        wbs = [
+            {"text": "tie",    "left": 284, "top": 264, "width": 23, "height": 14},
+            {"text": "sb/sth", "left": 309, "top": 264, "width": 57, "height": 20},
+            {"text": "up",     "left": 376, "top": 264, "width": 22, "height": 19},
+        ]
+        lines = _group_words_into_lines(wbs, y_tolerance_px=15)
+        sorted_wbs = [w for line in lines for w in line]
+        assert [w["text"] for w in sorted_wbs] == ["tie", "sb/sth", "up"]
+        # Same objects, same order
+        assert [id(w) for w in sorted_wbs] == [id(w) for w in wbs]