docs: Add missing features to comparison table

Added 6 new rows to competitor comparison:
- Attention Mechanisms (39 types, unique to RuVector)
- Hyperbolic Embeddings (Poincaré ball, unique)
- PostgreSQL Extension (pgvector-compatible, unique)
- SIMD Optimization (AVX-512/NEON)
- Metadata Filtering (common feature)
- Sparse Vectors (BM25/TF-IDF support)

🤖 Generated with [Claude Code](https://claude.com/claude-code)

Co-Authored-By: Claude <noreply@anthropic.com>

README.md

@@ -232,6 +232,12 @@ Production-validated metrics at hyperscale:
 | **Hyperedges** | ✅ | ❌ | ❌ | ❌ | ❌ |
 | **Self-Learning (GNN)** | ✅ | ❌ | ❌ | ❌ | ❌ |
 | **AI Agent Routing** | ✅ Tiny Dancer | ❌ | ❌ | ❌ | ❌ |
+| **Attention Mechanisms** | ✅ 39 types | ❌ | ❌ | ❌ | ❌ |
+| **Hyperbolic Embeddings** | ✅ Poincaré | ❌ | ❌ | ❌ | ❌ |
+| **PostgreSQL Extension** | ✅ pgvector-compatible | ❌ | ❌ | ❌ | ❌ |
+| **SIMD Optimization** | ✅ AVX-512/NEON | Partial | ✅ | ✅ | ❌ |
+| **Metadata Filtering** | ✅ | ✅ | ✅ | ✅ | ✅ |
+| **Sparse Vectors** | ✅ BM25/TF-IDF | ✅ | ✅ | ✅ | ❌ |
 | **Raft Consensus** | ✅ | ❌ | ✅ | ❌ | ❌ |
 | **Multi-Master Replication** | ✅ | ❌ | ❌ | ✅ | ❌ |
 | **Auto-Compression** | ✅ 2-32x | ❌ | ❌ | ✅ | ❌ |