Bake https://dl.yattee.stream/appcast.xml into SUFeedURL and align the
appcast template and Sparkle setup notes so the gh-pages branch (served
under the custom Cloudflare-CNAMEd domain) is the single place to look
up the feed. Domain is reserved as a generic distribution surface for
Sparkle today and AltStore / other channels later.
New Release-DeveloperID configuration gates Sparkle behind a SPARKLE
compile flag so the App Store Release build stays Sparkle-free. Adds
SPUStandardUpdaterController wrapper, Check for Updates menu command,
Advanced Settings section with beta channel toggle, and a Ruby script
plus GitHub Actions job that signs each release and publishes the
appcast to gh-pages for consumption by Sparkle and Homebrew cask.
