Fix IPA continuation: only process fully-bracketed cells, keep phrasal verb particles

Two fixes:
1. Step 5d now only treats cells as continuation when text is entirely
   inside brackets (e.g. "[n, nn]"). Cells with headwords outside brackets
   (e.g. "employee [im'ploi:]") are no longer overwritten.
2. fix_ipa_continuation_cell no longer skips grammar words like "down" —
   they are part of the headword in phrasal verbs like "close sth. down".

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

klausur-service/backend/grid_editor_api.py

@@ -1634,7 +1634,13 @@ async def _build_grid_core(session_id: str, session: dict) -> dict:
                     ct = cell.get("col_type", "")
                     if not ct.startswith("column_"):
                         continue
-                    cell_text = cell.get("text", "")
+                    cell_text = (cell.get("text") or "").strip()
+                    # Only treat as continuation when text is entirely
+                    # inside brackets — e.g. "[n, nn]", "[klaoz 'daun]".
+                    # Text like "employee [im'ploi:]" has a headword
+                    # OUTSIDE brackets and must NOT be overwritten.
+                    if not (cell_text.startswith('[') and cell_text.endswith(']')):
+                        continue
                     if not _text_has_garbled_ipa(cell_text):
                         continue
                     # Already has proper IPA brackets → already fixed