v0.10.3 — RRF search fusion + phrase-match rerank

Search quality: the keyword (bm25+recency) and semantic rankings are now fused with Reciprocal Rank Fusion (k=60) instead of an ad-hoc 0.5/0.5 score blend — robust to the bm25-vs-cosine scale mismatch. Plus a light rerank: an exact query phrase in a memory's summary lifts it to ~the top, all query terms present lifts it ~one rank slot. Tunable via MEM_RRF_K.