feat(phase3-4): Complete hybrid retrieval + LLM optimization pipeline
Phase 3: Hybrid Retrieval
- HybridRetriever: TF-IDF prefilter + semantic rerank + RRF fusion
- WikiScopedFilter: BFS wiki-graph traversal
- RetrievalRoute: Direct | WikiScoped | ReferenceOnly
- 10 unit tests
Phase 4: LLM Call Optimization
- ChunkOptimizer: unified pipeline (threshold + budget + dedup)
- ScoreThresholdFilter: configurable min_score (default 0.6)
- BudgetSelector: greedy selection within byte budget
- ShingleDeduplicator: Jaccard similarity dedup
- 8 unit tests
QueryRouter (Phase 3+4 Integration)
- Bridges WikiLinkGraph + HybridRetriever + ChunkOptimizer
- RouterConfig: max_hops, thresholds, budget, RRF weights
- WikiGraphBuilder: construct graph from markdown docs
- 11 unit tests
Integration Tests (it_phase3_phase4.rs)
- 19 end-to-end tests covering full pipeline
- Wiki-link parsing, graph traversal, route selection
- TF-IDF prefilter, RRF fusion, chunk optimization
- Edge cases (empty, no matches, config customization)
Total: 107 tests passing (was 32)
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