[Performance] Benchmark Suite and Profiling Setup

[avelino]

ChronDB has performance-sensitive operations (commit creation, history lookups, and diffs) but no standardized benchmarking framework.

Why
Benchmarking is crucial to identify regressions and validate optimizations like compression, pruning, or cache tuning.

What to do

• Implement reproducible benchmarks using Criterium or JMH interop.
• Measure P50/P95/P99 latency for:
  • save
  • get
  • get-at
  • history
  • scan
• Test with datasets of 1k, 100k, and 1M keys.
• Generate reports in Markdown and commit to benchmarks/.

Acceptance Criteria

• Benchmark suite runs automatically via CI.
• Metrics logged and stored per commit.
• Reports show baseline and deltas for each optimization.