Persona and group ids say which day, nothing more

An ObjectId carries its creation second and a per-process counter, so two
avatars made one after the other by the same login got ids a few counts
apart: a link between personas that every Mastodon client would have
seen. Avatar and Group now generate ids from the UTC day plus eight random
bytes (still valid ObjectIds), and a remote post's id is made from its
published time with a random tail, so posts page in the order they were
written rather than the order they arrived.

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

PrivaPub.Tests/Infrastructure/PrivacyIdsTests.cs

@@ -0,0 +1,40 @@
+using MongoDB.Bson;
+
+using PrivaPub.Infrastructure.Ids;
+using PrivaPub.Models.User;
+
+using GroupEntity = PrivaPub.Models.Group.Group;
+
+namespace PrivaPub.Tests.Infrastructure
+{
+	public class PrivacyIdsTests
+	{
+		[Fact]
+		public void Persona_ids_carry_only_the_day_and_share_nothing_else()
+		{
+			var first = (string)new Avatar().GenerateNewID();
+			var second = (string)new Avatar().GenerateNewID();
+
+			Assert.True(ObjectId.TryParse(first, out var a));
+			Assert.True(ObjectId.TryParse(second, out var b));
+			Assert.Equal(DateTime.UtcNow.Date, a.CreationTime);
+			Assert.Equal(a.CreationTime, b.CreationTime);
+			Assert.NotEqual(first[8..14], second[8..14]);
+			Assert.Equal(DateTime.UtcNow.Date, ObjectId.Parse((string)new GroupEntity().GenerateNewID()).CreationTime);
+		}
+
+		[Fact]
+		public void A_remote_post_id_sorts_by_its_published_time()
+		{
+			var older = PrivacyIds.At(new DateTime(2025, 1, 1, 0, 0, 0, DateTimeKind.Utc));
+			var newer = PrivacyIds.At(new DateTime(2025, 1, 2, 0, 0, 0, DateTimeKind.Utc));
+
+			Assert.True(string.CompareOrdinal(older, newer) < 0);
+			Assert.Equal(new DateTime(2025, 1, 1, 0, 0, 0, DateTimeKind.Utc), ObjectId.Parse(older).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));
+	}
+}