polish(verify): honest "no usable manifest" log (not "offline") for 404/corrupt

The no-manifest skip path fires for 404 (version not published) and corrupt
responses too, not only offline — the old "offline / no cache" wording misled
an online user on a version whose manifest isn't uploaded yet. From the final
5-dim CR (correctness leg, cosmetic-accuracy nit).

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>