Skip garbled IPA text in single-cell heading detection

Unbracketed IPA continuations like "ska:f – ska:vz" were falsely detected
as headings. Now _text_has_garbled_ipa() filters them out.

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

klausur-service/backend/grid_editor_api.py

@@ -736,6 +736,9 @@ def _detect_heading_rows_by_single_cell(
             text = (cell.get("text") or "").strip()
             if not text or text.startswith("["):
                 continue
+            # Skip garbled IPA without brackets (e.g. "ska:f – ska:vz")
+            if _text_has_garbled_ipa(text):
+                continue
             heading_row_indices.append(ri)
 
         for hri in heading_row_indices: