# Change: Real BM25 hybrid search and drop legacy SDK path

## Why

Delivery 2 shipped application-layer multi-path + sparse-lite lexical ranking. Product requirement is **true dense + BM25 hybrid retrieval**. Legacy `MilvusServiceClient` search path will be abandoned in this change; keep a single knowledge vector backend based on Milvus Java SDK v2 (`MilvusClientV2`) with BM25 function + `hybridSearch` + RRF.

## What Changes

- New hybrid collection schema: dense float vector + analyzed text + sparse BM25 output
- Write path inserts text for BM25 auto-sparsification and dense embedding
- Search path: single backend
  - dense mode: dense ANN only
  - hybrid mode: dense ANN + BM25 sparse ANN fused by RRFRanker
- Remove/disable legacy SDK `search` / `searchSimilarDocumentsWithSdk` knowledge path and mode routing (`sdk|spring|auto`)
- KnowledgeSearchPort adapter calls the single backend
- Config: collection name, search mode, rrf-k, path weights optional
- Tests for store mapping/fusion wiring; no dependency on live Zilliz in unit tests

## Non-goals

- Automatic full production reindex job UI
- Cross-encoder model rerank
- Keeping dual Spring-AI + SDK knowledge search modes

## Impact

- **Breaking for existing `biz` collection**: requires new hybrid collection + reindex
- Agent tool contract unchanged
- Interface: L2 internal retrieval backend

## Depends on

- Delivery 1 chunk identity
- Delivery 2 SearchPort (will replace sparse-lite hybrid implementation)