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/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));