Require API version-bump checks after the bootstrap release

Context

The release workflow currently runs the package bump check with:

bump_check: warn

at .github/workflows/release.yml:132-143.

The comment explains a valid bootstrap exception: the previous stable 0.20.0 bundle and alpha-0 are no longer parseable by the new compiler. It also says to change the check to require after the first stable new-compiler .tar.zst release.

Leaving this as a comment-only follow-up makes it easy for later releases to continue publishing despite an incorrect semantic version bump.

Suggested direction

After the first stable new-compiler release is available and parseable, change bump_check to require. If possible, make the transition version-aware so the exception automatically expires after the bootstrap version rather than relying on a later manual edit.

Acceptance criteria

• The bootstrap exception is limited to the specific release(s) that need it.
• All subsequent stable releases fail before publication when the API bump is insufficient.
• Release-candidate and pre-release behavior is documented and tested.
• The workflow emits a clear diagnostic containing the detected required bump.
• The obsolete bootstrap comment is removed or updated once enforcement is active.

Start at .github/workflows/release.yml:132-143 and inspect the existing package bump check, bootstrap comment, and release conditions. Determine how the bootstrap exception and stable, release-candidate, and pre-release cases are represented. Done means later stable releases require a sufficient API bump, diagnostics show the required bump, and the behavior is documented and tested.