Halve lesson gain when the bag has no textbook for the subject.

Locker and home do not count. A pack without the field keeps vanilla 0.5.

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

src/HSchool.Simulation/LessonLearningSystem.cs

@@ -8,8 +8,9 @@ namespace HSchool.Simulation;
 
 /// <summary>
 /// Grows skills for people who are actually in the lesson: at the room, not walking, not off
-/// doing something else, and with the assigned teacher standing there. The formula lives in
-/// <see cref="HSchool.Ai.LessonLearning"/>.
+/// doing something else, and with the assigned teacher standing there. A pupil without today's
+/// textbook in the bag learns at <see cref="BehaviorDef.LessonNoTextbookFactor"/>. The formula
+/// lives in <see cref="HSchool.Ai.LessonLearning"/>.
 /// </summary>
 internal static class LessonLearningSystem
 {
@@ -90,6 +91,13 @@ internal static class LessonLearningSystem
                     school.TryLogLessonOnce(personId, PersonLogTypes.LessonCold, lesson.Subject);
                 }
 
+                var textbookFactor = 1f;
+                if (person.IsStudent && !LessonLearning.HasTextbookInBag(person.Items, lesson.Subject))
+                {
+                    textbookFactor = rules.LessonNoTextbookFactor;
+                    school.TryLogLessonOnce(personId, PersonLogTypes.LessonNoTextbook, lesson.Subject);
+                }
+
                 IReadOnlyDictionary<string, float> taught = teacherSkills.TryGetValue(lesson.TeacherId, out var found)
                     ? found
                     : new Dictionary<string, float>(StringComparer.Ordinal);
@@ -115,7 +123,8 @@ internal static class LessonLearningSystem
                         hunger,
                         TraitOffset(catalog, traits, share.Skill),
                         teacherSkill,
-                        warmth);
+                        warmth,
+                        textbookFactor);
                 }
             });
     }