A clip fetched its range signed out, so a video behind YouTube's per-video "not a bot" wall could
never be clipped while its download went through with the user's cookie - and the failure copy told
the user to add a cookie the clip would not use. The clip asset is private by construction
(`clip.asset_ref` names its user), so `usable_clip_cookie` attaches the user's YouTube cookie on that
identity, and a failed range fetch now carries the download catalog's diagnosis
(`youtube.bot_wall` / `youtube.cookie_rejected`) instead of a bare category.
peter
f221c1962a