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>

tests/HSchool.Ai.Tests/LessonLearningTests.cs

@@ -1,4 +1,5 @@
 using HSchool.Content;
+using HSchool.People;
 
 namespace HSchool.Ai.Tests;
 
@@ -97,6 +98,40 @@ public class LessonLearningTests
 
         Assert.Equal(explicitWarm, implied);
     }
+
+    [Fact]
+    public void MissingTextbook_HalvesGain()
+    {
+        var withBook = LessonLearning.Gain(50, Math, 1, 0.05f, 1f, 1f, 0, 100, warmth: 1f, textbookFactor: 1f);
+        var without = LessonLearning.Gain(50, Math, 1, 0.05f, 1f, 1f, 0, 100, warmth: 1f, textbookFactor: 0.5f);
+
+        Assert.Equal((withBook - 50) * 0.5f, without - 50, precision: 5);
+    }
+
+    [Fact]
+    public void TextbookInBag_OnlyThatSubjectCounts()
+    {
+        var items = new[]
+        {
+            new InventoryItem("Textbook", null, 1f, ItemLocations.Bag, "Literature"),
+            new InventoryItem("Textbook", null, 1f, ItemLocations.Locker, "Mathematics"),
+        };
+
+        Assert.False(LessonLearning.HasTextbookInBag(items, "Mathematics"));
+        Assert.True(LessonLearning.HasTextbookInBag(items, "Literature"));
+    }
+
+    [Fact]
+    public void TextbookAtHome_DoesNotCount()
+    {
+        var items = new[]
+        {
+            new InventoryItem("Textbook", null, 1f, ItemLocations.Home, "Mathematics"),
+        };
+
+        Assert.False(LessonLearning.HasTextbookInBag(items, "Mathematics"));
+    }
+
     [Fact]
     public void MissingTeacherSkill_UsesRangeMin_AndStillGains()
     {