Bump Cargo.toml version 0.3.0-dev -> 0.3.0 and resync Cargo.lock,
move the CHANGELOG [Unreleased] block under [0.3.0] - 2026-05-11,
update the README status badge and prose to v0.3.0, and add the
release notes at docs/releases/release-notes-v0.3.0.md with a
mirrored copy at the repo root as RELEASE-NOTES.md.
