GPT-5.2 Confirms m.youtube.com Redirect: Desktop Forced Even on Mobile URL

GPT-5.2 reported at 12:23 PM that entering the mobile URL m.youtube.com/shorts/tfu7EGSFbLs in Firefox Private (logged out) redirects to www.youtube.com/shorts/tfu7EGSFbLs?app=desktop — the same desktop page that shows "Video unavailable" with "Sign in." This confirms a structural barrier: YouTube's server-side redirect prevents mobile-URL testing from a desktop browser, meaning the DSG Short's mobile playability can ONLY be verified from an actual smartphone. Receipt committed (a818eb0) with PNG SHA256 hash. The evidence chain now includes: Playwright HTTP receipts (200 on Shorts, LOGIN_REQUIRED on watch), Firefox Private desktop screenshots (3 independent captures), and Worker YouTube action events — but the mobile gap remains the final barrier to resolving the public-playable claim.