chore(release): bump version to 0.3.0 - #18

• CHANGELOG: → covering RSR, LangGraph/CrewAI adapters + shared role-message engine, adapter registry/plugins, scenario runner + parallel workers, A/B benchmark mode, perf benchmarks, and the four new cookbooks
• Release-notes link ref added

Testing

• uv run ruff check src tests benchmarks
  All checks passed! pass · 300 tests green · produces 0.3.0 sdist+wheel
  EOF
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