macOS: Sparkle beta update channel and release plumbing

[geekflyer]

Adds a menu toggle for the beta Sparkle feed, embeds stable/beta appcast URLs in Info.plist, extends release-mac.yml (git_ref, sparkle_channel) and release.sh (--beta) for a long-lived beta branch, and documents hosting appcast-beta.xml.

Made with Cursor

[claude]

--beta-mac is effectively a no-op at runtime.

resolvedFeedURLString() in SparkleUpdateChannel.swift reads ClipRelaySparkleFeedStable first, falling back to SUFeedURL only when that key is absent:

let stable = Bundle.main.object(forInfoDictionaryKey: "ClipRelaySparkleFeedStable") as? String
    ?? Bundle.main.object(forInfoDictionaryKey: "SUFeedURL") as? String

Since build_mac() always writes ClipRelaySparkleFeedStable (line 169), SUFeedURL is never used for the stable URL, so changing SPARKLE_INFO_DEFAULT_FEED here has no effect on which feed the app actually checks at runtime.

To make --beta-mac work as documented, it should also override SPARKLE_FEED_STABLE so the ClipRelaySparkleFeedStable plist key points at the beta feed — or the flag should be removed (the comment already notes "use menu toggle at runtime").

[claude]

Review summary

Overall the approach is clean: using SPUUpdaterDelegate.feedURLString(for:) to supply the runtime feed URL is the correct Sparkle 2.x pattern, and embedding both ClipRelaySparkleFeedStable / ClipRelaySparkleFeedBeta keys in Info.plist gives a nice compile-time-configurable default with a hardcoded fallback.

One bug found (inline): --beta-mac in build-all.sh only mutates SUFeedURL via SPARKLE_INFO_DEFAULT_FEED, but resolvedFeedURLString() reads ClipRelaySparkleFeedStable first and never reaches SUFeedURL in a normal build. The flag is a no-op — either drop it or have it also set SPARKLE_FEED_STABLE to point at the beta feed.

Everything else looks good: the clearFeedURLFromUserDefaults() call correctly evicts any stale URL written by the deprecated API before the delegate takes over, the appcast-update step correctly conditions on !inputs.prerelease (the raw input, not the computed release flag), and the --all --beta guard in release.sh prevents accidentally mixing Android and beta macOS releases.