Extract release source archive packaging

[m1ngsama]

Summary

• extract explicit release source archive creation into scripts/package_source_archive.sh
• reuse the source archive script from the release workflow and strict release preflight
• add regression coverage for source archive creation and wire release artifact tests to the real source archive path

Verification

• sh -n scripts/package_source_archive.sh
• workflow YAML parse for .github/workflows/release.yml and .github/workflows/ci.yml
• ./test_source_archive.sh
• ./test_release_artifact_gate.sh
• make script-test
• make release-check

Notes

This does not tag, publish, upload, release, or deploy. Production deployment and package-manager publishing remain manual.