PaperSpine5 release rollback

The immutable rollback point for this candidate is the commit referenced by tag v0.3.0-rc.1. Release ZIP files must never be replaced in place.

If the website is faulty, redeploy the preceding successful GitHub Pages artifact or revert only the website commit and rerun the Pages workflow. If an artifact is faulty, mark this prerelease as withdrawn and publish a new version with new filenames and checksums. Do not delete or overwrite evidence needed to explain the withdrawal.

PaperSpine V4 remains available from the repository root and its established dist/paperspine_version.json update source. V5 is opt-in, so rolling back the V5 page or prerelease does not require changing the V4 user-data identity.