Persist talk opinions and pick topics by weight.

A finished circle never set Tick's peopleChanged bit, so people.json kept pre-talk numbers until morning drift. PickTopic compared a 0..total roll to a 0..1 cursor, so gossip almost never changed the subject.

Co-authored-by: Cursor <cursoragent@cursor.com>

tests/HSchool.Simulation.Tests/MorningOpinionsTests.cs

@@ -63,6 +63,22 @@ public class MorningOpinionsTests
         Assert.True(after < before);
     }
 
+    [Fact]
+    public void DriftToZero_DropsTheRecord()
+    {
+        using var school = OpenStaffed();
+        var child = school.Roster!.People.First(person => person.IsStudent && !person.IsParent);
+        var classmate = school.Roster.People.First(person =>
+            person.IsStudent
+            && !person.Id.Equals(child.Id, StringComparison.Ordinal)
+            && !OpinionStore.FamilyMemberIds(school.Roster, child).Contains(person.Id));
+        var drift = school.Catalog!.BehaviorRules!.OpinionDriftPerMorning;
+        OpinionStore.Set(child, classmate.Id, Math.Min(2, drift));
+
+        Assert.True(MorningOpinions.Apply(school));
+        Assert.False(child.Opinions.ContainsKey(classmate.Id));
+    }
+
     private static void AdvanceTo(School school, DateTime until)
     {
         while (school.Clock.Time < until)