Add class-teacher slots on SchoolClass with owner-only assign/clear.

EOF

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

src/HSchool.People/Roster.cs

@@ -142,4 +142,6 @@ public sealed record SchoolClass(
     string Letter,
     string RoomId,
     int Capacity,
-    IReadOnlyList<string> PupilIds);
+    IReadOnlyList<string> PupilIds,
+    /// <summary>Hired teacher who is the class teacher, or null when the slot is empty.</summary>
+    string? ClassTeacherId = null);