Summary:
- bump the package version from 0.3.0-dev to 0.3.0
- move the unreleased changelog entries into the 0.3.0 section
- keep the current README messaging intact, including the disclaimer

Rationale:
- align the package metadata and release notes with the release tag
- complete the first step in the documented release checklist before publish

Validation:
- mix test
- mix docs
- mix hex.build

Co-authored-by: Codex <codex@openai.com>