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.Simulation/HealthConditionSystem.cs

@@ -3,24 +3,10 @@ using HSchool.People;
 namespace HSchool.Simulation;
 
 /// <summary>
-/// Ticks medical condition severity on the roster. Contagion and DiseaseDef outbreaks are later phases.
+/// Ticks medical conditions. DiseaseDef onset and stage curves live in <see cref="DiseaseSystem"/>.
+/// Kept so older call sites and phase-77 tests still have a named entry.
 /// </summary>
 internal static class HealthConditionSystem
 {
-    public static bool Apply(School school, double gameMinutes)
-    {
-        if (gameMinutes <= 0 || school.Roster is null)
-        {
-            return false;
-        }
-
-        var dayNumber = DateOnly.FromDateTime(school.Clock.Time).DayNumber;
-        var changed = false;
-        foreach (var person in school.Roster.People)
-        {
-            changed |= HealthConditions.Tick(person, school.PeopleSeed, dayNumber, gameMinutes);
-        }
-
-        return changed;
-    }
+    public static bool Apply(School school, double gameMinutes) => DiseaseSystem.Apply(school, gameMinutes);
 }