fix: verify PyPI release publication

[wesleysimplicio]

Summary

• pin the PyPI release job and Version Sync guard to the ubuntu-24.04 runner
• bound the release job runtime so a moving runner label cannot fail before the first step
• add a post-upload gate that verifies simplicio-mapper==<version> on PyPI
• compare the published wheel and sdist SHA-256 digests with the locally built artifacts
• retry only transient PyPI visibility failures; fail immediately on metadata or digest mismatches
• add focused workflow and verifier tests

Validation

• 12 passed focused pytest tests
• Ruff check passed for the new verifier and tests
• workflow YAML parsed successfully
• version-sync check passed for 0.26.19
• wheel and sdist built successfully; twine check dist/* passed

Closes #575