Everything on, phase 2: circle posts for everyone, private posts on signed refetch, browsers past SecureMode

Circles (owner decision 2026-10-04: fix them for compatibility):
- Mastodon 4.7 and GoToSocial drop a post that names none of their accounts, and a circle post named only the circle
  and its /flock. OutboxPublisher.Publish now sends each member a copy that also names that member in `cc`, on the
  activity and on the object, and names no other member. The Create, every Update (edit, poll, quote approval, policy,
  through the new PublishUpdate) and the Delete (StatusService.Remove now uses Publish) all go that way.
- UpdateOf renders with the post's group, so an Update keeps a circle post's `audience` and a community post's `Page`
  and title.
- A reply to a circle post stays in the circle, whichever client wrote it.
- A circle post can no longer quote a post that needs permission: asking would show the circle post to its author.

Posts that are not public, on refetch (SignedFetchAuthorizer.MayRead):
- Followers-only, direct and circle posts are served to a signed request from someone they were for, or from the
  instance actor of a server where one of them lives. That is a follower or an addressed account, an addressed
  account, or a member. Everyone else still gets 404.
- Once deleted they answer those readers 410. Mastodon deletes its copy when a refetch answers 404.
- A circle refetch names the requesting member, or the members on the requesting server, as the delivered copy did.
- /grunts/create-{id} serves the same.
- /peasants/{name}/whispers/{id}, a DM's `context`, was never routed. It is now the conversation's posts, for its
  participants only.

SecureMode lets browsers through to the redirect to the public page, instead of answering them 401.

653 tests pass.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01ELjqpznMFMNrJoJUj6K5p2

PrivaPub/Federation/Rendering/ActivityPubRenderer.cs

@@ -178,11 +178,11 @@ namespace PrivaPub.Federation.Rendering
 			return note;
 		}
 
-		public static JsonObject UpdateOf(PostEntity post, LocalActor author, string reason)
+		public static JsonObject UpdateOf(PostEntity post, LocalActor author, LocalActor group, string reason)
 		{
 			var note = post.Visibility == PostVisibility.Direct
 				? DirectNote(post, author, Array.Empty<(string, string)>(), post.ContextURI)
-				: Note(post, author, default, post.InReplyToURI);
+				: Note(post, author, group, post.InReplyToURI);
 			note["to"] = new JsonArray(post.To.Select(t => (JsonNode)t).ToArray());
 			note["cc"] = new JsonArray(post.Cc.Select(c => (JsonNode)c).ToArray());
 			return new JsonObject