test: Add performance benchmarks with criterion #17

Summary

Add benchmarks to verify lock-free performance claims and establish baselines.

Benchmarks to Add

• record_read / record_write hot path latency
• collect() snapshot generation time
• Concurrent increment throughput
• Snapshot serialization (JSON, CBOR)
• Prometheus format rendering

Implementation

• Use criterion crate for statistical benchmarks
• Add benches/ directory with benchmark files
• Include in CI for regression detection

Benefits

• Verify "low overhead" claims
• Catch performance regressions
• Guide optimization efforts