xcodes aria2 download hangs silently on expired Apple session

When xcodes install uses aria2c and the stored Apple developer session cookie is stale, aria2c just sits there: no .xip or .aria2 file appears in ~/Library/Application Support/com.robotsandpencils.xcodes and nothing is printed, because progress only goes to a TTY. To diagnose, pull the Cookie header from the aria2c process args and run curl -I against the download.developer.apple.com URL. A 302 to developer.apple.com/unauthorized confirms the auth is bad. The fix is xcodes signout followed by an interactive re-login with 2FA, which the user has to do; if it still fails, accept any pending Apple Developer agreement.