Add person-to-person disease contagion with outbreak notices.

Node/class contact rolls use pair+day seeds; vanilla respiratory diseases spread, and a threshold raises an info toast instead of per-sneeze noise.

Co-authored-by: Cursor <cursoragent@cursor.com>

src/HSchool.Content/PeopleDefValidator.cs

@@ -757,6 +757,12 @@ internal static class PeopleDefValidator
             throw new ContentLoadException(
                 $"BehaviorDef '{behavior.DefName}' diseaseVectorScale cannot be negative.");
         }
+
+        if (behavior.DiseaseOutbreakMinNewCases < 0)
+        {
+            throw new ContentLoadException(
+                $"BehaviorDef '{behavior.DefName}' diseaseOutbreakMinNewCases cannot be negative.");
+        }
     }
 
     private static void ValidateTopic(TopicDef topic, DefCatalog catalog)