v0.9.0 Search Integrations: Elasticsearch AsyncVectorIndex (elastic feature) over a hand-rolled reqwest client, plus cross-engine FederatedSearch (federation feature) with per-backend timeout and rank-based RRF fusion as the default. Includes review hardening: federated over-fetch (2k) + truncate to preserve RRF rank-credit, knn num_candidates cap (MAX_KNN_CANDIDATES=1000), ES error-body length cap with credential-safe truncation, and score-semantics documentation.