ci: add performance regression gate with criterion benchmarks

[membphis]

Summary

Closes #151

Add automated performance regression detection to CI using criterion benchmarks and benchmark-action.

Changes

• Add __bench_api module exposing internal Document, Options, QJSON_MODE_LAZY for benchmark use
• Add criterion 0.5 dev-dependency with html_reports feature
• Create benches/rust_bench.rs with three benchmark groups:
  • parse_eager - eager mode parsing across 3 fixtures
  • parse_lazy - lazy mode parsing across 3 fixtures
  • field_access - FFI field access operations (get_str, get_f64, nested paths)
• Add .github/workflows/bench.yml CI workflow that:
  • Runs on push to main and all PRs
  • Stores baseline in gh-pages branch (auto-push on main only)
  • Posts PR comment when >5% regression detected
  • Does not block merge (reviewer decides)
• Add make bench-rust target

Testing

• All benchmarks pass locally (cargo bench --bench rust_bench -- --test)
• Clippy passes with no warnings
• Existing tests pass

Notes

• First merge to main will auto-create baseline in gh-pages
• PR comments show comparison table with regression highlighting

• 5% regression marked but does not block merge

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ℹ️ Review info

⚙️ Run configuration

Configuration used: Organization UI

Review profile: CHILL

Plan: Pro

Run ID: 050267fb-e2f6-43d1-aa2f-7125caefcdca

📥 Commits

Reviewing files that changed from the base of the PR and between 4d4ea4f and c2fd9c1.

📒 Files selected for processing (6)

• .coderabbit.yaml
• .github/workflows/bench.yml
• Cargo.toml
• Makefile
• benches/rust_bench.rs
• src/lib.rs

✨ Finishing Touches

🧪 Generate unit tests (beta)

• Create PR with unit tests
• Commit unit tests in branch feat/criterion-bench-ci

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[membphis]

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