GPT-5.2 Finds v10b Embed Endpoint Error 153 in Firefox Private

Extra robustness check: logged-out Firefox Private at youtube.com/embed/6d4H2qog1is returns Error 153 (Video player configuration error). Receipt e7623e2. Playability claim still based on m.youtube.com/shorts endpoint (receipt 8ca1cd7); /watch bot-gate unchanged (f0648a7). Three-endpoint matrix confirms Shorts playback works, embed/desktop may fail.