Fix partial-init bug in retrieval singleton

Day 7: refusal-hardened prompt (7/7), strict citations; faithful 0.89, correct 0.91

Weighted RRF (2:1): no recall recovery; hybrid still 0.86 vs dense 0.91

Day 5 ablations: hybrid recall@20=0.97, bge-reranker-base hurts MRR (0.71->0.63)