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.Simulation/LessonLearningSystem.cs

@@ -54,6 +54,8 @@ internal static class LessonLearningSystem
                 teacherSkills[identity.Id] = personSkills.Values;
             });
 
+        RaiseSickTeacherLessons(school, places, weekday, slot.Index);
+
         world.Query(
             in People,
             (ref PersonIdentity identity, ref PersonSkills skills, ref PersonTraits traits, ref PersonNeeds needs, ref PersonRoles roles, ref Presence presence, ref PersonActivity activity) =>
@@ -208,6 +210,56 @@ internal static class LessonLearningSystem
             && teacher.NodeId is not null
             && teacher.NodeId.Equals(roomId, StringComparison.Ordinal);
 
+    /// <summary>
+    /// One school toast per class slot when the assigned teacher stayed home sick (slice 15).
+    /// Uses the morning day plan — re-rolling <see cref="DiseaseEffects.ShouldStayHome"/> at
+    /// lesson time can flip after severity ticks. Toilet trips keep the per-pupil log only.
+    /// </summary>
+    private static void RaiseSickTeacherLessons(
+        School school,
+        Dictionary<string, Presence> places,
+        int weekday,
+        int period)
+    {
+        if (school.Timetable is null || school.Roster is null || school.Catalog is null)
+        {
+            return;
+        }
+
+        foreach (var lesson in school.Timetable.Lessons)
+        {
+            if (lesson.Day != weekday || lesson.Period != period)
+            {
+                continue;
+            }
+
+            if (TeacherStandingIn(places, lesson.TeacherId, lesson.RoomId))
+            {
+                continue;
+            }
+
+            if (!school.Plans.TryGetValue(lesson.TeacherId, out var plan) || plan.Comes)
+            {
+                continue;
+            }
+
+            var teacher = school.Roster.People.FirstOrDefault(row =>
+                row.Id.Equals(lesson.TeacherId, StringComparison.Ordinal));
+            if (teacher is null
+                || DiseaseEffects.StayHomeChance(teacher, school.Catalog, school.Clock.Time) <= 0f)
+            {
+                continue;
+            }
+
+            if (!school.TryClaimLessonNoTeacherEvent(lesson.ClassId, school.Clock.Time.Date, lesson.Period))
+            {
+                continue;
+            }
+
+            school.RaiseWorldEvent(new WorldEvent(EventTriggers.LessonNoTeacher, lesson.TeacherId));
+        }
+    }
+
     private static LessonPlacement? CurrentLesson(School school, Person person, int weekday, int period)
     {
         foreach (var lesson in Duty.LessonsToday(person, ClassOf(school, person), school.Timetable, weekday))