Forwarded activities are believed as far as their origin vouches

A thread's server passes on what happens in it, signed with its own key: Mastodon forwards the replies to its
accounts' posts and their deletions, Friendica every activity in its threads. PrivaPub answered them 401, which also
tells a sender its signature failed. Now they get 202 and nothing in them is believed: a forwarded Create or Update is
taken as its object reads at the actor's origin, a Delete of a public or unlisted copy once that origin answers 404 or
410 (RemoteActorService.IsGone; FederationHttp remembers the status of a refusal), anything else is let go, and our own
activities coming back are ignored. A forwarded copy has its own dedupe key, so one that failed never hides the
author's own delivery.

A reply in the thread of someone followed here is kept, as Mastodon keeps them. Mastodon delivers a reply to the
followers of the account it answers; PrivaPub dropped those as unaddressed, which the pasture showed: the outsider's
reply its Mastodon scenario said was never delivered had been, and was thrown away.

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

PrivaPub/Federation/Actors/RemoteActorService.cs

@@ -20,6 +20,7 @@ namespace PrivaPub.Federation.Actors
 		Task<ForeignAvatar> GetActor(string actorUri, bool refresh, CancellationToken token);
 		Task<ForeignAvatar> GetActorByKeyId(string keyId, bool refresh, CancellationToken token);
 		bool KeyTemporarilyUnavailable(string keyId) => false;
+		Task<bool> IsGone(string uri, CancellationToken token) => Task.FromResult(false);
 		Task<string> ResolveHandle(string handle, CancellationToken token);
 	}
 
@@ -138,6 +139,18 @@ namespace PrivaPub.Federation.Actors
 
 		public bool KeyTemporarilyUnavailable(string keyId) => Origin.Of(keyId) != default && _http.FailedTemporarily(StripFragment(keyId));
 
+		// whether an object's origin says it is gone: 404 or 410 to our instance actor, or a Tombstone in its place
+		public async Task<bool> IsGone(string uri, CancellationToken token)
+		{
+			if (Origin.Of(uri) == default)
+				return false;
+			using var scope = HttpScope.Default("object");
+			var signer = await _localActors.GetInstanceActor(token);
+			var (status, fetched) = await _http.GetJsonStatus(uri, Accept, request => HttpSignatures.Sign(request, signer, body: null), token);
+			using (fetched)
+				return status is StatusCodes.Status404NotFound or StatusCodes.Status410Gone || fetched != default && Text(fetched.Root, "type") == "Tombstone";
+		}
+
 		public async Task<string> ResolveHandle(string handle, CancellationToken token)
 		{
 			var parts = handle?.TrimStart('@').Split('@');