feat(rag): hybrid multi-path search with RRF fusion

Add configurable hybrid mode on KnowledgeSearchPort that fuses dense unfiltered, dense filtered, and lexical ranks via RRF while preserving dense-compatible threshold scores. Archives Delivery 2 OpenSpec change.
2026-07-27 18:33:31 +08:00
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+# Decisions: rag-hybrid-search-rrf
+
+## Capability
+
+sm-flow + OpenSpec fallback; apply pre-authorized.
+
+## Depends
+
+Delivery 1 archived.
+
+## Grill (compressed, pre-authorized)
+
+- Q: Full BM25 schema now? A: No — sparse-lite + RRF first; schema rebuild follow-up.
+- Q: Default mode? A: dense default; hybrid opt-in.
+- Q: Threshold score? A: keep dense-compatible L2 mapping.
+
+## Design
+
+Hybrid paths: dense unfiltered + dense filtered + lexical rank over union; RRF fuse by evidenceKey.