A post named by its page is found as by its id

Pixelfed names one of our posts by the address of its page
(/@name/<post id>, the post's url) in its Like, Announce and their Undo,
so its likes were dropped as unknown objects. Before an activity is
handled, such a reference to a post of ours, as its object or the object
of the activity it undoes, is replaced by the post's id. Found by the
pasture's new Pixelfed peer.

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

PrivaPub.Tests/Federation/InboxGapTests.cs

@@ -42,6 +42,31 @@ namespace PrivaPub.Tests.Federation
 
 		PrivaPub.Models.Statistics.InteractionEvent Processed(string activity) => _harness.Ledger.Of("in").Last(e => e.Activity == activity);
 
+		// Pixelfed names our post by its page (/@name/<id>, the post's url) in its Like, Announce and their Undo: the post is
+		// found the same as by its id
+		[Fact]
+		public async Task A_like_naming_a_post_by_its_page_counts_and_its_undo_too()
+		{
+			var token = TestContext.Current.CancellationToken;
+			var (_, alice) = await _harness.Persona("alice");
+			var fan = new RemoteActor(_harness.Peer, "fan");
+			var post = (await _harness.Statuses.Publish(alice, new StatusDraft { Text = "a picture" }, token)).Post;
+			var page = alice.PostHtmlUrl(post.ID);
+			Assert.NotEqual(post.ObjectURI, page);
+			var like = new JsonObject { ["id"] = NewId(fan, "likes"), ["type"] = "Like", ["actor"] = fan.Id, ["object"] = page };
+
+			await _harness.Deliver(fan, "/human-centipede", like);
+			Assert.Equal("accepted", Processed("Like").Outcome);
+			Assert.Equal(1, (await DB.Default.Find<Post>().OneAsync(post.ID, token)).FavouritesCount);
+			await _harness.Deliver(fan, "/human-centipede", Activity(fan, "Undo", like));
+			Assert.Equal(0, (await DB.Default.Find<Post>().OneAsync(post.ID, token)).FavouritesCount);
+
+			// a page that is no post of ours stays as it is
+			var elsewhere = new JsonObject { ["id"] = NewId(fan, "likes"), ["type"] = "Like", ["actor"] = fan.Id, ["object"] = alice.PostHtmlUrl("000000000000000000000000") };
+			await _harness.Deliver(fan, "/human-centipede", elsewhere);
+			Assert.Equal(("dropped", "unknown-object"), (Processed("Like").Outcome, Processed("Like").Reason));
+		}
+
 		[Fact]
 		public async Task An_actor_update_refreshes_the_account_from_its_origin_even_when_its_updated_is_older()
 		{