Restore a published label version to the draft

Replaces mutable entity, product, and vault-label content with the selected immutable snapshot. Geo policies, entity membership and addresses, and vault assessments remain live and are not changed. The mutable draft belongs to the public label set; other label sets return a conflict. The restore is atomic and returns a conflict when one of those retained records references content that the selected snapshot would remove. Restoring schedules the resulting draft through the normal publication pipeline; it does not publish a new version itself.