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/PackPaths.cs

@@ -141,6 +141,9 @@ internal static class PackPaths
             case "events":
                 kind = DefKind.Event;
                 return true;
+            case "diseases":
+                kind = DefKind.Disease;
+                return true;
             default:
                 kind = default;
                 return false;