Add tests/perf_benchmark.rs with three side-by-side benchmarks that
compare old vs new approaches for the optimizations introduced in the
preceding commits:
- bench_label_insert_individual_vs_batch: measures N individual INSERTs
vs single multi-row INSERT (5k iterations, ~1.6x speedup)
- bench_string_building_old_vs_new: measures format!+push_str vs
writeln! (50k iterations, ~1.9x speedup)
- bench_prepare_vs_prepare_cached: measures prepare vs prepare_cached
(10k iterations, ~1.6x speedup)
Each benchmark verifies correctness (both approaches produce identical
output) and uses std::hint::black_box to prevent dead-code
elimination. Run with: cargo test --test perf_benchmark -- --nocapture
Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>
