feat: add Criterion benchmark suite for performance tracking

[alvinreal]

Adds a comprehensive benchmark suite using the criterion crate to track performance and detect regressions.

Benchmarks

Parsing (with throughput measurement)

• JSON: 100 / 1,000 / 10,000 records
• CSV: 100 / 1,000 / 10,000 rows
• YAML: 100 / 1,000 / 10,000 records

Serialization

• JSON (compact & pretty): 100 / 1,000 / 10,000 records
• CSV: 100 / 1,000 / 10,000 rows
• YAML: 100 / 1,000 / 10,000 records
• TOML: 100 / 1,000 records (smaller due to TOML's table structure)

Format Conversion (end-to-end)

• JSON → YAML: 100 / 1,000 / 10,000 records
• CSV → JSON: 100 / 1,000 / 10,000 rows

Mapping Operations

• Rename: single field rename across 100 / 1k / 10k records
• Filter: where .active == true across 100 / 1k / 10k records
• Complex: rename + set + drop + cast pipeline across 100 / 1k / 10k records

End-to-End

• JSON → JSON with mapping (rename + where filter): 100 / 1k / 10k records

CI Integration

Adds .github/workflows/bench.yml that:

• Runs on PRs to main
• Executes all benchmarks with --output-format bencher
• Uploads benchmark results + criterion HTML reports as artifacts

Usage

# Run all benchmarks
cargo bench

# Run specific benchmark group
cargo bench -- parse_json
cargo bench -- mapping_complex

# Quick benchmark (fewer iterations)
cargo bench -- --quick

Fixes #32