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.Content/PeopleDefs.cs

@@ -274,6 +274,12 @@ public sealed class BehaviorDef : Def
     /// <summary>Skill points a lesson adds per game hour, before traits and need state.</summary>
     public float LessonSkillPerHour { get; init; }
 
+    /// <summary>
+    /// Multiplier when the bag has no textbook for this lesson. Locker and home do not count.
+    /// A pack without the field keeps vanilla half-gain so the catalog still loads.
+    /// </summary>
+    public float LessonNoTextbookFactor { get; init; } = 0.5f;
+
     /// <summary>Inclusive range of extra commute minutes rolled per person per day.</summary>
     public int CommuteSlackMin { get; init; }