P7: feeds read as accounts, and PieFed's post Move

Lemmy 1.0's multi-communities and PieFed's feeds (`type: Feed`) resolve as accounts: the feed's `following` is kept
once a day with its counts, each community in it is read in turn, and `/api/v1/accounts/:id/following` lists them.
Lemmy's feed carries the instance's key and its text in `description`; both are taken as they are. Following a feed
answers 422 until the owner decides how a feed is followed.

A post moved to another community (PieFed's Move{object: post, origin, target}), relayed by the community it leaves,
goes there with its thread; a move into a community we host is not taken.

Checked in the pasture: a Lemmy 1.0 feed and a PieFed feed list their communities (ours among them); a PieFed post moved
from its web code arrives moved, with its comment.

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

PrivaPub.Tests/Federation/InboxGapTests.cs

@@ -412,6 +412,48 @@ namespace PrivaPub.Tests.Federation
 			Assert.Equal(0, (await DB.Default.Find<Post>().OneAsync(reply.ID, token)).FavouritesCount);
 		}
 
+		// PieFed moves a post to another community, and the community it leaves relays the Move: the post's thread goes with it
+		[Fact]
+		public async Task A_post_moved_to_another_community_takes_its_thread_along()
+		{
+			var token = TestContext.Current.CancellationToken;
+			var (root, alice) = await _harness.Persona("alice");
+			var community = new RemoteActor(_harness.Peer, "cats", type: "Group");
+			var other = new RemoteActor(_harness.Peer, "dogs", type: "Group");
+			var poster = new RemoteActor(_harness.Peer, "poster");
+			await _harness.Follows.Follow(root, new FollowForm { AvatarId = alice.Id, Target = community.Id }, token);
+			await DB.Default.Update<Following>().Match(f => f.AvatarId == alice.Id).Modify(f => f.State, FollowState.Accepted).ExecuteAsync(token);
+			async Task<Post> Announced(JsonObject note)
+			{
+				note["audience"] = community.Id;
+				_harness.Peer.Serve(new Uri(IdOf(note)).AbsolutePath, note.ToJsonString());
+				await _harness.Deliver(community, "/human-centipede", Activity(community, "Announce", Create(poster, note)));
+				return await DB.Default.Find<Post>().Match(p => p.ObjectURI == IdOf(note)).ExecuteSingleAsync(token);
+			}
+			var page = PublicNote(poster, "<p>misplaced</p>", community.Id);
+			page["type"] = "Page";
+			page["name"] = "misplaced";
+			var post = await Announced(page);
+			var comment = PublicNote(poster, "<p>a comment</p>", community.Id);
+			comment["inReplyTo"] = IdOf(page);
+			var answer = await Announced(comment);
+			var move = new JsonObject
+			{
+				["id"] = NewId(poster, "moves"), ["type"] = "Move", ["actor"] = poster.Id, ["object"] = post.ObjectURI, ["origin"] = community.Id, ["target"] = other.Id
+			};
+
+			// only the community it leaves speaks for it
+			await _harness.Deliver(community, "/human-centipede", Activity(community, "Announce", new JsonObject
+			{
+				["id"] = NewId(poster, "moves"), ["type"] = "Move", ["actor"] = poster.Id, ["object"] = post.ObjectURI, ["origin"] = other.Id, ["target"] = community.Id
+			}));
+			Assert.Equal(("dropped", "unsupported"), (Processed("Announce").Outcome, Processed("Announce").Reason));
+			await _harness.Deliver(community, "/human-centipede", Activity(community, "Announce", move));
+			Assert.Equal(("accepted", "moved"), (Processed("Announce").Outcome, Processed("Announce").Reason));
+			Assert.Equal(other.Id, (await DB.Default.Find<Post>().OneAsync(post.ID, token)).AudienceURI);
+			Assert.Equal(other.Id, (await DB.Default.Find<Post>().OneAsync(answer.ID, token)).AudienceURI);
+		}
+
 		// Lemmy's moderators lock a community's post and ban members, and the community relays both inside its Announce
 		// (G-0006): a locked post takes no reply, a banned persona posts nothing there, until the Undo
 		[Fact]