fix(sub-columns): convert relative word positions to absolute coords for split

Word 'left' values in ColumnGeometry.words are relative to the content
ROI (left_x), but geo.x is in absolute image coordinates. The split
position was computed from relative word positions and then compared
against absolute geo.x, resulting in negative widths and no splits on
real data. Pass left_x through to _detect_sub_columns to bridge the
two coordinate systems.

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

klausur-service/backend/tests/test_cv_vocab_pipeline.py

@@ -1307,6 +1307,29 @@ class TestSubColumnDetection:
 
         assert len(result) == 1
 
+    def test_sub_column_split_with_left_x_offset(self):
+        """Word 'left' values are relative to left_x; geo.x is absolute.
+
+        Real-world scenario: left_x=195, EN column at geo.x=310.
+        Page refs at relative left=115-157, vocab words at relative left=216.
+        Without left_x, split_x would be ~202 (< geo.x=310) → negative width → no split.
+        With left_x=195, split_abs = 202 + 195 = 397, which is between geo.x(310)
+        and geo.x+geo.width(748) → valid split.
+        """
+        content_w = 1469
+        left_x = 195
+        page_refs = [self._make_word(115, "p.59"), self._make_word(157, "p.60"),
+                     self._make_word(157, "p.61")]
+        vocab = [self._make_word(216, f"word{i}") for i in range(40)]
+        all_words = page_refs + vocab
+        geo = self._make_geo(x=310, width=438, words=all_words, content_w=content_w)
+
+        result = _detect_sub_columns([geo], content_w, left_x=left_x)
+
+        assert len(result) == 2, f"Expected 2 columns, got {len(result)}"
+        assert result[0].word_count == 3
+        assert result[1].word_count == 40
+
 
 class TestCellsToVocabEntriesPageRef:
     """Test that page_ref cells are mapped to source_page field."""