Deliveries to followers carry a digest of them, per server

Owner decision of 2026-10-06 (FEP-8fcf). A persona's delivery addressed to its followers carries a signed
Collection-Synchronization header naming its followers, its roll-call (…/groupies/roll-call) and the digest of its
accepted followers on the receiving server only. The roll-call answers a signed request with the persona's followers
on the signer's server and nobody else's.

Mastodon gives every Undo{Follow} it sends after reading a roll-call the same id (…#follows//undo), so a second one
looked like a copy: an Undo of a Follow that comes again while the follow it ends exists again is now kept once per
follow.

Checked live (scenarios/followsync.sh): Mastodon drops a follow PrivaPub lost, and undoes one it lost itself.

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

PrivaPub/Federation/Signing/HttpSignatures.cs

@@ -17,7 +17,8 @@ namespace PrivaPub.Federation.Signing
 
 		public static string Digest(byte[] body) => "SHA-256=" + Convert.ToBase64String(SHA256.HashData(body));
 
-		public static void Sign(HttpRequestMessage request, LocalActor signer, byte[] body)
+		// extra: further headers the request carries, signed with the rest (FEP-8fcf's Collection-Synchronization)
+		public static void Sign(HttpRequestMessage request, LocalActor signer, byte[] body, params (string Name, string Value)[] extra)
 		{
 			var date = DateTime.UtcNow.ToString("r", CultureInfo.InvariantCulture);
 			var signed = new List<(string Name, string Value)>
@@ -35,6 +36,12 @@ namespace PrivaPub.Federation.Signing
 				signed.Add(("digest", digest));
 			}
 
+			foreach (var (name, value) in extra)
+			{
+				request.Headers.TryAddWithoutValidation(name, value);
+				signed.Add((name.ToLowerInvariant(), value));
+			}
+
 			var signingString = string.Join("\n", signed.Select(h => $"{h.Name}: {h.Value}"));
 			using var rsa = RSA.Create();
 			rsa.ImportFromPem(signer.PrivateKeyPem);