Moves carry follows over; first DMs to Lemmy 0.19 and Mbin go as ChatMessage

Two owner decisions of 2026-10-05, both recorded in ROADMAP:
- After a verified Move the personas following the old account follow the new one, in the same lists, and a mute or
  block of the old account carries over, as Mastodon does it.
- A direct message to one account on a server whose NodeInfo names Lemmy before 1.0 or Mbin goes as a ChatMessage,
  the one place PrivaPub decides by a server's software (invariant 17). G-0008 is closed.

Mbin addresses its private messages to the recipient's profile page, so a Create addressed to a persona's /@name now
reaches the persona. Checked live: moves 8/8, Lemmy 0.19 30/30, Mbin 26/26 with messages both ways. The software
theory runs alone, since every test's peer shares 127.0.0.1.

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

PrivaPub.Tests/Federation/ChatMessageSoftwareTests.cs

@@ -0,0 +1,75 @@
+using MongoDB.Entities;
+
+using PrivaPub.ClientModels.Post;
+using PrivaPub.Models.Post;
+using PrivaPub.Tests.Support;
+
+using System.Text.Json.Nodes;
+
+namespace PrivaPub.Tests.Federation
+{
+	// alone in its collection: every test's peer is 127.0.0.1, and the server row it writes decides for whoever writes there
+	[Trait("Category", "Integration")]
+	[Xunit.Collection(nameof(Exclusive))]
+	public sealed class ChatMessageSoftwareTests : IAsyncLifetime
+	{
+		Harness _harness;
+
+		public async ValueTask InitializeAsync()
+		{
+			Assert.SkipUnless(MongoFixture.Enabled, MongoFixture.Skip);
+			_harness = await Harness.Start();
+		}
+
+		public async ValueTask DisposeAsync()
+		{
+			if (_harness != default)
+				await _harness.DisposeAsync();
+		}
+
+		static string NewId(RemoteActor actor, string kind) => $"{new Uri(actor.Id).GetLeftPart(UriPartial.Authority)}/{kind}/{Guid.NewGuid():N}";
+
+		// the one decision taken by a server's software (owner decision 2026-10-05): Lemmy before 1.0 and Mbin get a
+		// ChatMessage even from a persona answering a Note; Lemmy 1.0 a Note
+		[Theory]
+		[InlineData("lemmy", "0.19.20", "ChatMessage")]
+		[InlineData("mbin", "1.10.1", "ChatMessage")]
+		[InlineData("lemmy", "1.0.0-beta.2", "Note")]
+		public async Task A_direct_message_to_lemmy_before_1_0_or_mbin_is_a_chat_message(string software, string version, string sent)
+		{
+			var token = TestContext.Current.CancellationToken;
+			var (_, alice) = await _harness.Persona("alice");
+			var sender = new RemoteActor(_harness.Peer, "pm");
+			var host = new Uri(sender.Id).Host;
+			await DB.Default.DeleteAsync<PrivaPub.Models.Jobs.RemoteInstance>(i => i.Host == host);
+			await DB.Default.SaveAsync(new PrivaPub.Models.Jobs.RemoteInstance { Host = host, Software = software, SoftwareVersion = version }, token);
+			try
+			{
+				var id = NewId(sender, "messages");
+				await _harness.Deliver(sender, "/human-centipede", new JsonObject
+				{
+					["id"] = NewId(sender, "activities/create"), ["type"] = "Create", ["actor"] = sender.Id, ["to"] = new JsonArray(alice.Uri),
+					["object"] = new JsonObject
+					{
+						["id"] = id, ["type"] = "Note", ["attributedTo"] = sender.Id, ["to"] = new JsonArray(alice.Uri),
+						["content"] = "<p>psst</p>", ["published"] = DateTime.UtcNow.ToString("O")
+					}
+				});
+				var message = await DB.Default.Find<Post>().Match(p => p.ObjectURI == id).ExecuteSingleAsync(token);
+
+				var answer = await _harness.Statuses.Publish(alice, new PrivaPub.Domain.Statuses.StatusDraft
+				{
+					Text = "psst back", Visibility = PostVisibility.Direct, ConversationId = message.ConversationId
+				}, token);
+
+				Assert.True(answer.Ok, answer.Error);
+				var create = (await _harness.Outgoing(sender.Id + "/inbox")).Last(a => a["type"]!.GetValue<string>() == "Create");
+				Assert.Equal(sent, create["object"]!["type"]!.GetValue<string>());
+			}
+			finally
+			{
+				await DB.Default.DeleteAsync<PrivaPub.Models.Jobs.RemoteInstance>(i => i.Host == host);
+			}
+		}
+	}
+}