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/School.cs

@@ -119,6 +119,9 @@ public sealed class School : IDisposable
     /// <summary>Last slot <see cref="AttendanceSystem"/> observed — detects leaving a lesson to finalize absents.</summary>
     internal DaySlot? LastAttendanceSlot { get; set; }
 
+    /// <summary>Calendar day of the last DiseaseDef onset pass (one roll per person per day).</summary>
+    internal int LastDiseaseDay { get; set; } = int.MinValue;
+
     internal Dictionary<string, DayPlan> Plans { get; } = new(StringComparer.Ordinal);
 
     internal Queue<string> DecisionQueue { get; } = new();
@@ -219,6 +222,7 @@ public sealed class School : IDisposable
         PlanDay = null;
         LastDecisionSlot = null;
         LastAttendanceSlot = null;
+        LastDiseaseDay = int.MinValue;
         Plans.Clear();
         DecisionQueue.Clear();
         LoggedActivity.Clear();