Add DiseaseDef diseases with weather vectors and excused absence.

Vanilla ships several diseases whose stages drive lesson gain, stay-home
rolls, and illness attendance; cold/rain/snow raise onset via BehaviorDef scale.

src/HSchool.Content/PeopleDefValidator.cs

@@ -733,6 +733,12 @@ internal static class PeopleDefValidator
             throw new ContentLoadException(
                 $"BehaviorDef '{behavior.DefName}' lessonLateMinutes cannot be negative.");
         }
+
+        if (behavior.DiseaseVectorScale < 0f)
+        {
+            throw new ContentLoadException(
+                $"BehaviorDef '{behavior.DefName}' diseaseVectorScale cannot be negative.");
+        }
     }
 
     private static void ValidateTopic(TopicDef topic, DefCatalog catalog)