A sender's digest of its followers here is compared and mended

FEP-8fcf, received. When a delivery's Collection-Synchronization header digests the sender's followers on PrivaPub
otherwise than the personas following it, a job reads the list the header names (on the sender's origin, signed by the
instance actor): a follow the list leaves out ends, only when the list is the one the digest describes; a request it
lists is taken as accepted; a persona it lists that follows nothing there sends Undo{Follow}, as Mastodon does. Each
claiming delivery is compared once.

Checked live (scenarios/followsync.sh, now 15 checks): PrivaPub ends a follow Mastodon lost and undoes one only
Mastodon remembered.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01LsXgEaXee4GCU1hwYgPJXw

PrivaPub/Federation/Actors/FollowersSynchronization.cs

@@ -55,6 +55,23 @@ namespace PrivaPub.Federation.Actors
 		public static string HeaderValue(LocalActor actor, string digest) =>
 			$"collectionId=\"{actor.Followers}\", url=\"{RollCall(actor)}\", digest=\"{digest}\"";
 
+		// what a Collection-Synchronization header claims, written as a Signature header's parameters; default when it
+		// names no collection, list or digest
+		public static SynchronizationClaim Claim(string actorUri, string header)
+		{
+			var values = new Dictionary<string, string>(StringComparer.Ordinal);
+			foreach (var part in (header ?? "").Split(',', StringSplitOptions.TrimEntries | StringSplitOptions.RemoveEmptyEntries))
+			{
+				var equals = part.IndexOf('=');
+				if (equals > 0)
+					values[part[..equals].Trim()] = part[(equals + 1)..].Trim().Trim('"');
+			}
+			return values.TryGetValue("collectionId", out var collection) && values.TryGetValue("url", out var url) && values.TryGetValue("digest", out var digest)
+				&& digest.Length == 64 && digest.All(Uri.IsHexDigit)
+				? new SynchronizationClaim(actorUri, collection, url, digest.ToLowerInvariant())
+				: default;
+		}
+
 		// whether the activity, or the object it carries, is addressed to the actor's followers
 		public static bool ForFollowers(JsonObject activity, LocalActor actor) =>
 			Addressed(activity, actor.Followers) || activity["object"] is JsonObject inner && Addressed(inner, actor.Followers);