Bump Cargo.toml version 0.2.1-dev -> 0.2.1 and resync Cargo.lock,
move the CHANGELOG [Unreleased] block under [0.2.1] - 2026-05-11,
update the README status badge and prose to v0.2.1, and add the
release notes at docs/releases/release-notes-v0.2.1.md with a
mirrored copy at the repo root as RELEASE-NOTES.md.
