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

EOF

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

src/HSchool.People/ClassTeachers.cs

@@ -0,0 +1,82 @@
+namespace HSchool.People;
+
+public enum ClassTeacherError
+{
+    None,
+    UnknownSchool,
+    UnknownClass,
+    UnknownPerson,
+    NotTeacher,
+}
+
+public sealed record ClassTeacherOutcome(ClassTeacherError Error, Roster Roster);
+
+/// <summary>
+/// Assigns or clears the class-teacher slot on a <see cref="SchoolClass"/>. Not a
+/// <see cref="PositionDef"/> — any hired teacher may cover several classes.
+/// </summary>
+public static class ClassTeachers
+{
+    public static ClassTeacherOutcome UnknownSchool() =>
+        new(ClassTeacherError.UnknownSchool, new Roster([], [], []));
+
+    public static ClassTeacherOutcome Assign(Roster roster, string classId, string personId)
+    {
+        ArgumentNullException.ThrowIfNull(roster);
+        ArgumentNullException.ThrowIfNull(classId);
+        ArgumentNullException.ThrowIfNull(personId);
+
+        var index = IndexOf(roster.Classes, classId);
+        if (index < 0)
+        {
+            return new ClassTeacherOutcome(ClassTeacherError.UnknownClass, roster);
+        }
+
+        var person = roster.People.FirstOrDefault(candidate => candidate.Id.Equals(personId, StringComparison.Ordinal));
+        if (person is null)
+        {
+            return new ClassTeacherOutcome(ClassTeacherError.UnknownPerson, roster);
+        }
+
+        if (!person.IsStaff || !string.Equals(person.Position, Staffing.TeacherPosition, StringComparison.Ordinal))
+        {
+            return new ClassTeacherOutcome(ClassTeacherError.NotTeacher, roster);
+        }
+
+        return Replace(roster, index, personId);
+    }
+
+    public static ClassTeacherOutcome Clear(Roster roster, string classId)
+    {
+        ArgumentNullException.ThrowIfNull(roster);
+        ArgumentNullException.ThrowIfNull(classId);
+
+        var index = IndexOf(roster.Classes, classId);
+        if (index < 0)
+        {
+            return new ClassTeacherOutcome(ClassTeacherError.UnknownClass, roster);
+        }
+
+        return Replace(roster, index, null);
+    }
+
+    private static ClassTeacherOutcome Replace(Roster roster, int index, string? teacherId)
+    {
+        var classes = roster.Classes.ToArray();
+        classes[index] = classes[index] with { ClassTeacherId = teacherId };
+        return new ClassTeacherOutcome(ClassTeacherError.None, roster with { Classes = classes });
+    }
+
+    private static int IndexOf(IReadOnlyList<SchoolClass> classes, string classId)
+    {
+        for (var i = 0; i < classes.Count; i++)
+        {
+            if (classes[i].Id.Equals(classId, StringComparison.Ordinal))
+            {
+                return i;
+            }
+        }
+
+        return -1;
+    }
+}

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);