Remote posts take their timeline place on arrival, and an unedited one has no edit date

Found by the first live run against GoToSocial:

- A remote post's id came from its published second plus random bytes, so a reply arriving in the same second as
  the post it answers could sort under it, and a late arrival landed behind a client's since_id. A post published
  within the last hour now gets an id for its arrival; backfill keeps its published time.
- NoteParser.Time returned default(DateTime) for a missing "updated", so every remote post was stored as edited in
  year one. Migration _006 clears the stored ones.
- published now carries milliseconds, so peers that derive ids from it (GoToSocial, Mastodon) keep our posts in
  order within a second.

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

PrivaPub.Tests/Federation/NoteParserTests.cs

@@ -54,6 +54,7 @@ namespace PrivaPub.Tests.Federation
 			Assert.Equal("a cat", media.Description);
 			Assert.Equal(new[] { 0.5f, -1f }, media.Focus);
 			Assert.Equal(new DateTime(2026, 9, 30, 10, 0, 0, DateTimeKind.Utc), note.Published);
+			Assert.Null(note.Updated);
 		}
 
 		[Fact]

PrivaPub.Tests/Federation/RendererTests.cs

@@ -57,7 +57,7 @@ namespace PrivaPub.Tests.Federation
 				Language = "en",
 				Mentions = new() { new PostMention { ActorURI = "https://m.example/users/bob", Handle = "@bob@m.example" } },
 				Tags = new() { "cats" },
-				CreationDate = new DateTime(2026, 9, 30, 17, 45, 12, DateTimeKind.Utc)
+				CreationDate = new DateTime(2026, 9, 30, 17, 45, 12, 345, DateTimeKind.Utc)
 			};
 
 			var note = ActivityPubRenderer.Note(post, Alice, default, "https://m.example/users/bob/statuses/1");
@@ -67,7 +67,7 @@ namespace PrivaPub.Tests.Federation
 			Assert.Equal("<p><strong>On &lt;cats&gt;</strong></p><p>body</p>", note["content"]!.GetValue<string>());
 			Assert.Equal("On <cats>", note["summary"]!.GetValue<string>());
 			Assert.True(note["sensitive"]!.GetValue<bool>());
-			Assert.Equal("2026-09-30T17:45:12Z", note["published"]!.GetValue<string>());
+			Assert.Equal("2026-09-30T17:45:12.345Z", note["published"]!.GetValue<string>());
 			Assert.Equal(new[] { ActivityPubRenderer.Public }, note["to"]!.AsArray().Select(n => n!.GetValue<string>()));
 			Assert.Equal(new[] { Alice.Followers, "https://m.example/users/bob" }, note["cc"]!.AsArray().Select(n => n!.GetValue<string>()));
 			var tags = note["tag"]!.AsArray();

PrivaPub.Tests/Infrastructure/PrivacyIdsTests.cs

@@ -33,6 +33,22 @@ namespace PrivaPub.Tests.Infrastructure
 			Assert.Equal(new DateTime(2025, 1, 1, 0, 0, 0, DateTimeKind.Utc), ObjectId.Parse(older).CreationTime);
 		}
 
+		[Fact]
+		public void A_post_arriving_live_sorts_after_everything_already_stored()
+		{
+			var local = ObjectId.GenerateNewId().ToString();
+			var reply = PrivacyIds.Arrived(DateTime.UtcNow.AddSeconds(-30));
+			var skewed = PrivacyIds.Arrived(DateTime.UtcNow.AddYears(50));
+
+			Assert.True(string.CompareOrdinal(local, reply) < 0);
+			Assert.True(string.CompareOrdinal(reply, skewed) < 0);
+			Assert.True(ObjectId.Parse(skewed).CreationTime < DateTime.UtcNow.AddMinutes(1));
+		}
+
+		[Fact]
+		public void A_backfilled_post_keeps_its_published_time() =>
+			Assert.Equal(new DateTime(2025, 1, 1, 0, 0, 0, DateTimeKind.Utc), ObjectId.Parse(PrivacyIds.Arrived(new DateTime(2025, 1, 1, 0, 0, 0, DateTimeKind.Utc))).CreationTime);
+
 		[Fact]
 		public void A_published_time_in_the_future_is_clamped() =>
 			Assert.True(ObjectId.Parse(PrivacyIds.At(DateTime.UtcNow.AddYears(50))).CreationTime < DateTime.UtcNow.AddDays(2));