A public reply names the author it answers in cc

A reply whose text had lost the @name of the author it answers (decePub
prefills it, a reader may delete it) was addressed to nobody on the
author's server. PrivaPub delivered it to the author's inbox anyway, but
GoToSocial keeps only what is addressed to someone there, so the reply
never appeared under the post. A public or unlisted reply to a remote
post now names its parent's author in cc, as Pleroma does; the reply
already says whom it answers, so nothing new is revealed. The parent
author's actor id is kept on the reply (InReplyToActorURI).

Found by decePub's end-to-end tests on the town.

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

PrivaPub/Federation/Rendering/ActivityPubRenderer.cs

@@ -146,6 +146,11 @@ namespace PrivaPub.Federation.Rendering
 			};
 			if (group is { IsCircle: false })
 				cc.Add(group.Uri);
+			// a public reply names the author it answers in cc even when the @name was deleted from its text, as Pleroma does:
+			// GoToSocial keeps only what is addressed to someone there, and the reply already says whom it answers
+			if (post.Visibility is PostVisibility.Public or PostVisibility.Unlisted && !string.IsNullOrEmpty(post.InReplyToActorURI)
+				&& !mentions.Any(m => m.GetValue<string>() == post.InReplyToActorURI))
+				cc.Add(post.InReplyToActorURI);
 
 			var note = NoteBody(post, author, to, cc, inReplyTo);
 			if (group != default)