2.2 Hybrid Retrieval

Implement dense retrieval using the vector database (cosine similarity or dot product). - Implement sparse retrieval using BM25. - Combine results using Reciprocal Rank Fusion (RRF): `score(d) = sum_r 1 / (k + rank_r(d))` where k is typically 60 and the sum is over all retrieval methods. - Retrieve