feat: Cross-encoder search reranking with MiniLM-L-6

Phase 6.5: Pipeline telemetry, search UX fixes, latency profiling

Phase 6.3: Bug B fix + documentation finalization

Phase 6.1+6.2+6.3: Wire all 37 features into caller + health endpoint

Phase 6: LightGBM reranker integration (37-feature schema, 141-tree model)

Phase 4 complete + Phase 4.5 instrumentation foundation

Phase 3 complete: Hybrid Semantic Search pipeline