Update README with latest benchmark results

[rendro]

Summary

• Replace outdated performance claims ("Sub-16ms recall") with actual benchmarked numbers (50ms store, 103ms recall at 1K items)
• Update comparison table from feature-based (OpenMemory/mcp-memory-service) to benchmark-based (ChromaDB/Mem0/MCP Memory Svc) with real R@1, Recency@1, and dedup metrics
• Update Performance section with full benchmark comparison table

Test plan

• No code changes, documentation only