RAG++ SOTA Improvements

Technical documentation for state-of-the-art performance optimizations in RAG++ retrieval engine. 1. [Overview](#overview) 2. [Architecture](#architecture) 3. [SIMD Acceleration (P0)](#simd-acceleration-p0) 4. [Scalar Quantization - SQ8 (P1.1)](#scalar-quantization---sq8-p11) 5. [Product Quantization - PQ (P1.2)](#product-quantization---pq-p12) 6. [Parallel HNSW Construction (P2)](#parallel-hnsw-construction-p2) 7. [Hybrid Search - Dense + Sparse (P3)](#hybrid-search---dense--sparse-p3) 8. [Hybrid Query Engine (P4)](#hybrid-query-engine-p4) 9. [Benchmarks](#benchmarks) 10. [API Reference](#api-reference) 11. [Migration Guide](#migration-guide) RAG++ now includes production-ready optimizations for high-performance retrieval: | Improvement | Benefit | Trade-off | |-------------|---------|-----------| | **SIMD (AVX2)** | 4-8x faster distance computation | x86_64 only | | **SQ8** | 4x memory reduction | <1% recall loss | | **PQ** | 32-128x memory reduction | 5-20% recall loss | | **Parallel HNSW** | 2-4x faster index build | More memory during build | | **Hybrid Search** | +10-20% recall on keyword queries | ~50% more latency | 1. **Additive Changes**: All existing APIs remain unchanged 2. **Opt-in Performance**: New features are explicitly enabled 3. **Measurable Trade-offs**: Each optimization has documented recall/latency impact 4. **Composable**: Features can be combined (e.g., SQ8 + Hybrid)