Add server benchmark client

[adsharma]

Summary

• enable multi-write support for the embedded Ladybug system database
• add a bench_client binary for read, write, and mixed workload benchmarks
• report per-run throughput and p99 latency as CSV

Validation

• bash build.sh
• benchmark matrix run locally for 1, 2, 4, and 8 threads across read, write, and mixed workloads