ADR-004: Multi-Encoder RRF Fusion

Status: Accepted

Decision

• Core QueryRouteClassifier never auto-queries specialized collections (G4).
• RetrieverOrchestrator runs per-plan ANN; per-plan RERANK; merge via merge_rrf (G8).
• Cross-collection dedupe by source_chunk_id or (document_id, path) (G32).
• Single-plan failures are best-effort skipped (G14).