Conversations are keyed by who is in them, and circles stay home

S8: a direct message joins a conversation only when its participants are
exactly that conversation's members, found through a new
DmGroup.ParticipantsKey (a hash of the sorted members). A remote context
no longer decides anything: it let anyone who knew a conversation's
context post into it, and joining by context while dropping a participant
would have shown a reply to someone it was not addressed to. A context is
kept only when it is on the author's origin. Sending a DM to the same
people again reuses their conversation instead of opening a new one.

S9: Group.Kind is Circle or Community. A circle is not a federated actor:
its actor, collections, WebFinger and inbox answer 404, a remote Follow is
refused, and posts in it are IsLocalOnly - never delivered, never in an
outbox, never served. Communities keep today's behaviour until P4.
Migration _003 makes every existing group a circle, marks their posts
local-only and backfills the conversation keys.

End-to-end inbox tests sign real deliveries from a fake peer: a context
injection, a forged activity id, a note attributed to someone else, a
cross-origin object, a bad signature, junk bodies and a Follow of a circle.

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

PrivaPub.Tests/Support/RemoteActor.cs

@@ -0,0 +1,65 @@
+using Microsoft.AspNetCore.Http;
+using Microsoft.AspNetCore.Http.Features;
+
+using PrivaPub.Federation.Signing;
+
+using System.Globalization;
+using System.Security.Cryptography;
+using System.Text;
+using System.Text.Json.Nodes;
+
+namespace PrivaPub.Tests.Support
+{
+	public sealed class RemoteActor
+	{
+		readonly RSA _key = RSA.Create(2048);
+
+		public RemoteActor(Peer peer, string name, string origin = default)
+		{
+			Name = $"{name}{Guid.NewGuid():N}"[..20];
+			Id = $"{origin ?? peer.A}/users/{Name}";
+			peer.Serve($"/users/{Name}", Document().ToJsonString());
+		}
+
+		public string Name { get; }
+		public string Id { get; }
+		public string KeyId => Id + "#main-key";
+
+		public JsonObject Document() => new()
+		{
+			["id"] = Id,
+			["type"] = "Person",
+			["preferredUsername"] = Name,
+			["inbox"] = Id + "/inbox",
+			["followers"] = Id + "/followers",
+			["publicKey"] = new JsonObject
+			{
+				["id"] = KeyId,
+				["owner"] = Id,
+				["publicKeyPem"] = _key.ExportSubjectPublicKeyInfoPem()
+			}
+		};
+
+		public HttpRequest Post(string host, string path, JsonNode activity)
+		{
+			var body = Encoding.UTF8.GetBytes(activity.ToJsonString());
+			var date = DateTimeOffset.UtcNow.ToString("r", CultureInfo.InvariantCulture);
+			var digest = HttpSignatures.Digest(body);
+			var signingString = $"(request-target): post {path}\nhost: {host}\ndate: {date}\ndigest: {digest}";
+			var signature = Convert.ToBase64String(_key.SignData(Encoding.UTF8.GetBytes(signingString), HashAlgorithmName.SHA256, RSASignaturePadding.Pkcs1));
+
+			var context = new DefaultHttpContext();
+			context.Request.Method = "POST";
+			context.Request.Host = new HostString(host);
+			context.Request.Path = path;
+			context.Features.Get<IHttpRequestFeature>().RawTarget = path;
+			context.Request.Headers["Date"] = date;
+			context.Request.Headers["Digest"] = digest;
+			context.Request.Headers["Content-Type"] = "application/activity+json";
+			context.Request.Headers["Signature"] = $"keyId=\"{KeyId}\",algorithm=\"rsa-sha256\",headers=\"(request-target) host date digest\",signature=\"{signature}\"";
+			context.Request.Body = new MemoryStream(body);
+			context.Request.ContentLength = body.Length;
+			return context.Request;
+		}
+	}
+}