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/ocr_pipeline_api.py

@@ -700,7 +700,7 @@ async def detect_columns(session_id: str):
         cached["_content_bounds"] = (left_x, right_x, top_y, bottom_y)
 
         # Split sub-columns (e.g. page references) before classification
-        geometries = _detect_sub_columns(geometries, content_w)
+        geometries = _detect_sub_columns(geometries, content_w, left_x=left_x)
 
         # Phase B: Content-based classification
         regions = classify_column_types(geometries, content_w, top_y, w, h, bottom_y,