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The stable channel compares versions; the testing channel offers the staging
build of another commit. The channel switch is the starter's only, not the
simulator's.

StarterNG/Application/Abstractions/IReleaseFeed.cs

@@ -0,0 +1,28 @@
+using System.Threading;
+using System.Threading.Tasks;
+using StarterNG.Domain.Settings;
+
+namespace StarterNG.Application.Abstractions;
+
+/// <summary>
+/// A published build of the starter: its tag, the page it is downloaded from and
+/// the commit it was built from - the only way to tell rolling builds apart.
+/// </summary>
+public sealed record ReleaseInfo(string Tag, string PageUrl, string? Commit = null);
+
+/// <summary>
+/// What asking for the newest release came back with. No answer - offline, rate
+/// limited, timed out - is kept apart from an answer of "nothing released", so a
+/// failed check is not remembered as a done one.
+/// </summary>
+public sealed record ReleaseLookup(bool Answered, ReleaseInfo? Release)
+{
+    public static ReleaseLookup NoAnswer { get; } = new(false, null);
+
+    public static ReleaseLookup Found(ReleaseInfo? release) => new(true, release);
+}
+
+public interface IReleaseFeed
+{
+    Task<ReleaseLookup> LatestAsync(UpdateChannel channel, CancellationToken cancellationToken = default);
+}