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

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

Phase 6.5: Documentation finalization

Phase 6.5 Day 1: Real Qdrant cosine scores (A1) + verification timestamp (A2)

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 5: Cold-Start Onboarding + UI Redesign

Phase 4 complete + Phase 4.5 instrumentation foundation

docs: Update tracker - Phase 3.5 fully complete, all integration done

docs: Add Phase 3.5 Turso ArXiv Metadata DB, update infra status

Phase 3 complete: Hybrid Semantic Search pipeline