Fix FAISS search() API compatibility for newer versions

faiss >= 1.9 requires pre-allocated output arrays passed explicitly as
positional args: index.search(x, k, distances, labels).
The old two-return-value form broke on the HF Space runtime.

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Co-Authored-By: Devin <158243242+devin-ai-integration[bot]@users.noreply.github.com>

retrieval.py

@@ -57,7 +57,11 @@ def retrieve(query: str,
     query_embedding = model.encode([query], normalize_embeddings=True)
     query_embedding = np.array(query_embedding, dtype='float32')
 
-    scores, indices = index.search(query_embedding, top_k)
+    # Pre-allocate output arrays for FAISS compatibility across API versions
+    distances = np.empty((1, top_k), dtype='float32')
+    labels = np.empty((1, top_k), dtype='int64')
+    index.search(query_embedding, top_k, distances, labels)
+    scores, indices = distances, labels
 
     results = []
     for score, idx in zip(scores[0], indices[0]):