Fix IPA continuation: skip words with inline IPA, recover emptied cells

Three fixes:
1. fix_ipa_continuation_cell: when headword has inline IPA like
   "beat [bˈiːt] , beat, beaten", only generate IPA for uncovered
   words (beaten), not words already shown (beat). When bracket is
   at end like "the Highlands [ˈhaɪləndz]", return inline IPA directly.
2. Step 5d: recover garbled IPA from word_boxes when Step 5c emptied
   the cell text (e.g. "[n, nn]" → "").
3. Added 2 tests for inline IPA behavior (35 total).

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

klausur-service/backend/grid_editor_api.py

@@ -1798,7 +1798,13 @@ async def _build_grid_core(session_id: str, session: dict) -> dict:
                         continue
                     cell_text = (cell.get("text") or "").strip()
                     if not cell_text:
-                        continue
+                        # Step 5c may have emptied garbled IPA cells like
+                        # "[n, nn]" — recover text from word_boxes.
+                        wb_texts = [w.get("text", "")
+                                    for w in cell.get("word_boxes", [])]
+                        cell_text = " ".join(wb_texts).strip()
+                        if not cell_text:
+                            continue
 
                     is_bracketed = (
                         cell_text.startswith('[') and cell_text.endswith(']')