Wire sick-teacher stay-home into cancelled lessons.

Heavy flu keeps the assigned teacher off the day plan; pupils get 48/73 no-teacher marks and a one-shot LessonCancelled notice instead of silent empty rooms.

src/HSchool.Content/EventDefValidator.cs

@@ -17,6 +17,7 @@ internal static class EventDefValidator
         EventTriggers.DirectorSummon,
         EventTriggers.ParentMeeting,
         EventTriggers.DiseaseOutbreak,
+        EventTriggers.LessonNoTeacher,
     };
 
     private static readonly HashSet<string> Actions = new(StringComparer.Ordinal)

src/HSchool.Content/EventDefs.cs

@@ -15,6 +15,7 @@ public static class EventTriggers
     public const string DirectorSummon = "directorSummon";
     public const string ParentMeeting = "parentMeeting";
     public const string DiseaseOutbreak = "diseaseOutbreak";
+    public const string LessonNoTeacher = "lessonNoTeacher";
 }
 
 public static class EventActions