release: prepare the v0.3.0 source milestone

[noctem-o]

Prepares the tree for a v0.3.0 source release. Docs only.

Changes

• CHANGELOG.md: turns Unreleased into 0.3.0, with a short summary of what landed since v0.2.0: the CLI, portable verification in Rust, Python, and Go, LogWriter as the only write path, verified-replay projections, SQLite L0, checkpoint evaluation, and ADRs 0003–0010. Adds the missing 0.2.0 entry. The status history moved from CLAUDE.md sits unchanged at the end.
• README.md: the release coordinates point at v0.3.0 and say that magpie --version prints the crate version, 0.1.0.
• SECURITY.md: no longer names each release, so it won't go stale at the next tag.

Crate versions, the log format, and the standing policies are unchanged. Like v0.2.0, this is a source milestone. It is not the paused C3–C5 public pre-alpha.

After merge (owner): create the v0.3.0 release on the merge commit. The README's v0.3.0 link is a 404 until then.

Validation: git diff --check, python tools/check_release_metadata.py, and cargo package -p magpie-log --locked pass. A script confirmed the moved changelog text is byte-for-byte identical. cargo test wasn't run: the change is docs only, and no test reads these files.

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https://claude.ai/code/session_01EB1exYkc8Px1iG85NNWzxc

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📥 Commits

Reviewing files that changed from the base of the PR and between 7534af2 and a06daab.

📒 Files selected for processing (3)

• CHANGELOG.md
• README.md
• SECURITY.md

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