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/Defs.cs

@@ -26,6 +26,7 @@ public enum DefKind
     Orientation,
     AffinityRules,
     Event,
+    Disease,
 }
 
 /// <summary>Shared JSONC fields. Kind comes from the folder under <c>defs/</c>, not from the file.</summary>