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.Ai/LessonLearning.cs

@@ -1,10 +1,12 @@
 using HSchool.Content;
+using HSchool.People;
 
 namespace HSchool.Ai;
 
 /// <summary>
 /// How much a lesson adds to one skill this step. Hungry or cold learns worse; trait offsets and
-/// the teacher's subject skill scale the rate. Missing warmth is treated as 1.
+/// the teacher's subject skill scale the rate. Missing warmth is treated as 1. A missing textbook
+/// uses <see cref="BehaviorDef.LessonNoTextbookFactor"/>.
 /// </summary>
 public static class LessonLearning
 {
@@ -45,6 +47,28 @@ public static class LessonLearning
         return count == 0 ? 0f : total / count;
     }
 
+    /// <summary>
+    /// True when a bag instance carries this lesson's <see cref="InventoryItem.Subject"/>.
+    /// Locker and home do not count — the pupil is already in class.
+    /// </summary>
+    public static bool HasTextbookInBag(IReadOnlyList<InventoryItem> items, string subject)
+    {
+        ArgumentNullException.ThrowIfNull(items);
+        ArgumentException.ThrowIfNullOrWhiteSpace(subject);
+
+        foreach (var item in items)
+        {
+            if (item.Location.Equals(ItemLocations.Bag, StringComparison.Ordinal)
+                && item.Subject is not null
+                && item.Subject.Equals(subject, StringComparison.Ordinal))
+            {
+                return true;
+            }
+        }
+
+        return false;
+    }
+
     public static float Gain(
         float current,
         SkillDef skill,
@@ -54,7 +78,8 @@ public static class LessonLearning
         float hunger,
         int traitOffset,
         float teacherSkill,
-        float warmth = 1f)
+        float warmth = 1f,
+        float textbookFactor = 1f)
     {
         ArgumentNullException.ThrowIfNull(skill);
         var delta = share
@@ -63,7 +88,8 @@ public static class LessonLearning
             * NeedFactor(hunger)
             * NeedFactor(warmth)
             * TraitFactor(traitOffset)
-            * TeacherFactor(teacherSkill);
+            * TeacherFactor(teacherSkill)
+            * textbookFactor;
         return Math.Clamp(current + delta, skill.Range.Min, skill.Range.Max);
     }
 }