Subject URI

Yields the full canonical URI of a subject as a string — `publisher/package@version/name`. Authors typically use this inside SetTransformation iteration to derive per-input deep links, source-package paths, audit-trail attribution, or cross-package indexes that need to know where each matched instance came from. Returns undefined when the source expression doesn't yield a SubjectKanonak (or a ReferenceKanonak, which is supported as a convenience and produces the URI-string form of its target).