Perf Baseline Manager

It enforces one baseline JSON per version, storing them under a predictable structure and preventing the clutter of duplicate or conflicting files. You'd reach for this when consolidating benchmark runs, comparing metrics between releases, or just keeping your perf data organized. The constraint is simple but matters: overwrite old baselines for the same version instead of piling up files. It records both metrics and environment metadata, so you're not flying blind when numbers shift. The docs reference a canonical contract in perf-requirements.md, which suggests this is part of a broader testing workflow rather than a standalone experiment.