docs: update reference results to c7i.xlarge × 2 placement group

Update README reference results to reflect the new two-host benchmark setup (c7i.xlarge × 2, cluster placement group, eu-central-1):

- Hardware: 2 × AWS c7i.xlarge (4 vCPU, 8 GB RAM each), cluster placement group
- Latency: p50 decision 304 µs / proxy 302 µs / nginx 235 µs
- Throughput: 195 000 RPS for both simple and complex policies
- Remove tiktoken/LLM row (not supported)