M3: what each handler did with an activity

Arrival carries a verdict that handlers set with one line before an existing return
(Arrival.Drop, Reject, Accept, About), so no handler signature changes. InboxProcessor
records it as an 'in' event, with the time taken, the wait since the inbox accepted it, the
attempt, the audience (from the post's visibility, or else the activity's addressing), the
local actor kind, the object's age for updates, deletes and reactions, and the FEP features
of a delivered object. It also records types nothing handles (unknown-type), actors that
cannot be loaded (deferred) and handler failures.

Drop reasons: fetch-failed, unparseable, misattributed, duplicate, deleted, not-addressed,
not-followed, not-public, not-visible, not-deleted, unknown-object, unknown-recipient,
cross-origin, unsupported. Accepted sub-reasons: stored, poll-vote, edit, refresh,
actor-refresh, actor-delete, removed, tombstone-only, auto-accepted, pending,
follow-answer, quote-answer, quote-granted, undone, reaction, reported. Rejected: blocked,
ignored, quote-refused.

A circle's traffic is private and kindless, and a stranger posting into one is just
not-addressed, so the event store cannot reveal a circle.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01ELjqpznMFMNrJoJUj6K5p2

PrivaPub/Federation/Inbox/Handlers/JoinHandler.cs

@@ -30,11 +30,19 @@ namespace PrivaPub.Federation.Inbox.Handlers
 		{
 			var objectUri = Id(activity["object"]);
 			if (objectUri == default || !objectUri.StartsWith(_localActors.BaseAddress + "/", StringComparison.OrdinalIgnoreCase))
+			{
+				Arrival.Drop("unknown-object");
 				return;
+			}
 			var post = await _dbEntities.Posts.Match(p => p.ObjectURI == objectUri && !p.IsFederatedCopy && !p.DeletedAt.HasValue).ExecuteFirstAsync(token);
 			var owner = post == default ? default : await _localActors.FindById(LocalActorKind.Person, post.GroupUserId, token);
 			if (owner == default || string.IsNullOrEmpty(actor.InboxURL))
+			{
+				Arrival.Drop("unknown-object");
 				return;
+			}
+			Arrival.About(post.ObjectType ?? "Note", post.Visibility, post.CreationDate);
+			Arrival.Reject("ignored");
 			await _delivery.Enqueue(owner, new[] { actor.InboxURL }, new JsonObject
 			{
 				["@context"] = ActivityPubRenderer.ActivityStreams,