[codex] Add benchmark threshold guidance

[bobzhu94]

Summary

• Document how maintainers should compare before/after parser benchmark output.
• Add guidance for noisy local benchmark results without adding dependencies.
• Update README, roadmap, and changelog references so the maintenance path is discoverable.

Why

This closes #6 by turning the parser benchmark from a bare command into a repeatable review signal for parser-heavy changes.

Validation

• npm test

Closes #6