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/Federation/Rendering/ActivityPubRenderer.cs

@@ -109,17 +109,25 @@ namespace PrivaPub.Federation.Rendering
 			var mentions = post.Mentions.Select(m => (JsonNode)m.ActorURI).ToArray();
 			var (to, cc) = post.Visibility switch
 			{
+				PostVisibility.Circle when group != default => (new JsonArray(group.Uri, group.Flock), new JsonArray(mentions)),
 				PostVisibility.Unlisted => (new JsonArray(author.Followers), new JsonArray(mentions.Prepend(Public).ToArray())),
 				PostVisibility.FollowersOnly => (new JsonArray(author.Followers), new JsonArray(mentions)),
 				PostVisibility.Direct => (new JsonArray(mentions), new JsonArray()),
 				_ => (new JsonArray(Public), new JsonArray(mentions.Prepend(author.Followers).ToArray()))
 			};
-			if (group != default)
+			if (group is { IsCircle: false })
 				cc.Add(group.Uri);
 
 			var note = NoteBody(post, author, to, cc, inReplyTo);
 			if (group != default)
 				note["audience"] = group.Uri;
+			else if (!string.IsNullOrEmpty(post.AudienceURI))
+				note["audience"] = post.AudienceURI;
+			if (group is { IsCircle: false } && string.IsNullOrEmpty(inReplyTo))
+			{
+				note["type"] = "Page";
+				note["name"] = post.Title ?? Headline(post);
+			}
 			return note;
 		}
 
@@ -141,6 +149,13 @@ namespace PrivaPub.Federation.Rendering
 			return note;
 		}
 
+		static string Headline(PostEntity post)
+		{
+			var text = System.Text.RegularExpressions.Regex.Replace(post.ContentHtml ?? post.Text ?? string.Empty, "<[^>]+>", " ");
+			text = WebUtility.HtmlDecode(System.Text.RegularExpressions.Regex.Replace(text, "\\s+", " ")).Trim();
+			return text.Length <= 100 ? text : text[..97] + "...";
+		}
+
 		static JsonObject NoteBody(PostEntity post, LocalActor author, JsonArray to, JsonArray cc, string inReplyTo)
 		{
 			var content = post.ContentHtml ?? Html(post.Text);