Communities follow FEP-1b12, circles federate to their members only

Communities:
- a post addressed to a community (to, cc or audience) is accepted
  according to its posting policy - followers, anyone, or moderators -
  and GroupDistributor announces the whole activity with `audience` to
  the community's followers, plus the object for new posts so Mastodon
  shows them; updates and deletes of community content are announced too;
- top-level posts are Pages with a name (the title, or a headline from
  the text); /flock counts members, /wardens lists moderators;
- a Mastodon client posts into a community by mentioning it, or into a
  remote group, which sets `audience`;
- an Announce of an activity from a remote group a persona follows (Lemmy)
  is followed through: the object is fetched from its own origin, kept with
  its AudienceURI, and fanned out to the group's local followers; updates
  are applied in place and deletes checked against the origin.

Circles stop being local-only: an undiscoverable Group actor whose follows
are all requests the owner approves; posts addressed to the circle and its
/flock and delivered to members' own inboxes, never announced, never
public; a remote member's post into the circle is accepted from members
only. SignedFetchAuthorizer serves circle posts and collections only to a
signed request from a member or a member server's instance actor - 404 for
anyone else. Circles never surface in search, lookups, mentions, account
ids or profile pages.

Federation:SecureMode requires a valid signature on every GET under
/peasants except the instance actor. Group forms take a posting policy.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_012CzABvBkbcFqoHdmi8b9WB

PrivaPub.Tests/Support/Harness.cs

@@ -41,6 +41,7 @@ namespace PrivaPub.Tests.Support
 			Remote = new RemoteActorService(Peer.Http(cache), Local, cache, Db);
 			Delivery = new DeliveryService(Db, Queue);
 			Fanout = new Fanout(Db);
+			Groups = new GroupDistributor(Delivery);
 			RemotePosts = new RemotePosts(Db, Local, Remote, new NoBlocks(), Queue);
 			Receiver = new InboxReceiver(Local, Remote, Queue, new NoBlocks(), NullLogger<InboxReceiver>.Instance);
 			Processor = new InboxProcessor(Remote, new IActivityHandler[]
@@ -50,10 +51,10 @@ namespace PrivaPub.Tests.Support
 				new RejectHandler(Db, Local),
 				new UndoHandler(Db, Local),
 				new LikeHandler(Db),
-				new AnnounceHandler(Db, Local, RemotePosts, Fanout),
-				new CreateHandler(Db, Local, Remote, Delivery, new NoBlocks(), Fanout, RemotePosts),
-				new DeleteHandler(Db, Local, Remote, Delivery),
-				new UpdateHandler(Db, Local, Remote),
+				new AnnounceHandler(Db, Local, RemotePosts, Fanout, Remote),
+				new CreateHandler(Db, Local, Remote, Delivery, new NoBlocks(), Fanout, RemotePosts, Groups),
+				new DeleteHandler(Db, Local, Remote, Delivery, Groups),
+				new UpdateHandler(Db, Local, Remote, Groups),
 				new FlagHandler(Db, Local)
 			}, NullLogger<InboxProcessor>.Instance);
 			Follows = new FollowService(Db, Local, Remote, Delivery, new KeyLocalizer<GenericRes>(), NullLogger<FollowService>.Instance);
@@ -61,7 +62,7 @@ namespace PrivaPub.Tests.Support
 			Outbox = new OutboxPublisher(Db, Local, Delivery);
 			Media = new MediaService(new StaticOptions<MediaOptions>(new MediaOptions { Root = Path.Combine(Path.GetTempPath(), $"privapub-media-{Guid.NewGuid():N}") }),
 				Local, default, NullLogger<MediaService>.Instance);
-			Statuses = new StatusService(Db, Local, Remote, Delivery, Content, Outbox, Fanout, Media);
+			Statuses = new StatusService(Db, Local, Remote, Delivery, Content, Outbox, Fanout, Media, Groups);
 			Posts = new PostsService(Db, Local, Statuses, new KeyLocalizer<GenericRes>(), NullLogger<PostsService>.Instance);
 			Timelines = new TimelineService(Db, new KeyLocalizer<GenericRes>());
 			Relationships = new RelationshipService(Db, Follows, Delivery);
@@ -82,6 +83,7 @@ namespace PrivaPub.Tests.Support
 		public PostsService Posts { get; }
 		public StatusService Statuses { get; }
 		public MediaService Media { get; }
+		public GroupDistributor Groups { get; }
 		public Fanout Fanout { get; }
 		public RemotePosts RemotePosts { get; }
 		public TimelineService Timelines { get; }