Harden release script gates, cover them with tests

[DaniusKalv]

Summary

• Require a packaging/source-build commit status before creating a new source tag. CI posts it through a new reusable commit-status.yml on the commit it built, closing the window where the source branch moves between a green run and the tag.
• Check required tools up front and name the package providing each.
• Scope CI lookup to the build workflow and report each lookup failure distinctly.
• Cover every release guard with a stubbed selftest, 26 cases.
• Fold repeated error exits into one helper and align the release summary output.