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.People/DiseaseEffects.cs

@@ -0,0 +1,163 @@
+using HSchool.Content;
+
+namespace HSchool.People;
+
+/// <summary>
+/// Reads DiseaseDef stages for lesson gain, stay-home rolls and warmth decay.
+/// Contagion between people is phase 79.
+/// </summary>
+public static class DiseaseEffects
+{
+    public static bool TryStage(DiseaseDef disease, float severity, out DiseaseStage stage)
+    {
+        ArgumentNullException.ThrowIfNull(disease);
+        DiseaseStage? best = null;
+        foreach (var candidate in disease.Stages)
+        {
+            if (severity + 1e-6f < candidate.MinSeverity)
+            {
+                continue;
+            }
+
+            if (best is null || candidate.MinSeverity >= best.MinSeverity)
+            {
+                best = candidate;
+            }
+        }
+
+        if (best is null)
+        {
+            stage = null!;
+            return false;
+        }
+
+        stage = best;
+        return true;
+    }
+
+    public static bool IsIncubated(HealthCondition condition, DiseaseDef disease, DateTime now)
+    {
+        ArgumentNullException.ThrowIfNull(condition);
+        ArgumentNullException.ThrowIfNull(disease);
+        var elapsed = (DateTime.SpecifyKind(now, DateTimeKind.Utc) - condition.StartedAt).TotalDays;
+        return elapsed >= disease.IncubationDays;
+    }
+
+    /// <summary>Product of active post-incubation lesson factors (1 when healthy).</summary>
+    public static float LessonLearningFactor(Person person, DefCatalog catalog, DateTime now)
+    {
+        ArgumentNullException.ThrowIfNull(person);
+        ArgumentNullException.ThrowIfNull(catalog);
+        var list = person.Conditions;
+        if (list is null || list.Count == 0)
+        {
+            return 1f;
+        }
+
+        var factor = 1f;
+        foreach (var condition in list)
+        {
+            if (!catalog.Diseases.TryGetValue(condition.DefName, out var disease) || disease.Abstract)
+            {
+                continue;
+            }
+
+            if (!IsIncubated(condition, disease, now) || !TryStage(disease, condition.Severity, out var stage))
+            {
+                continue;
+            }
+
+            factor *= stage.LessonLearningFactor;
+        }
+
+        return factor;
+    }
+
+    /// <summary>Highest stay-home chance among incubated conditions.</summary>
+    public static float StayHomeChance(Person person, DefCatalog catalog, DateTime now)
+    {
+        ArgumentNullException.ThrowIfNull(person);
+        ArgumentNullException.ThrowIfNull(catalog);
+        var list = person.Conditions;
+        if (list is null || list.Count == 0)
+        {
+            return 0f;
+        }
+
+        var chance = 0f;
+        foreach (var condition in list)
+        {
+            if (!catalog.Diseases.TryGetValue(condition.DefName, out var disease) || disease.Abstract)
+            {
+                continue;
+            }
+
+            if (!IsIncubated(condition, disease, now) || !TryStage(disease, condition.Severity, out var stage))
+            {
+                continue;
+            }
+
+            if (stage.StayHomeChance > chance)
+            {
+                chance = stage.StayHomeChance;
+            }
+        }
+
+        return chance;
+    }
+
+    /// <summary>
+    /// Deterministic per person+day: whether illness keeps them off campus today.
+    /// </summary>
+    public static bool ShouldStayHome(Person person, DefCatalog catalog, int peopleSeed, DateTime now)
+    {
+        var chance = StayHomeChance(person, catalog, now);
+        if (chance <= 0f)
+        {
+            return false;
+        }
+
+        if (chance >= 1f)
+        {
+            return true;
+        }
+
+        var dayNumber = DateOnly.FromDateTime(DateTime.SpecifyKind(now, DateTimeKind.Utc)).DayNumber;
+        var roll = Seed.Mix(peopleSeed, person.Id, dayNumber, Seed.DiseaseSalt);
+        var unit = (roll & int.MaxValue) / (float)int.MaxValue;
+        return unit < chance;
+    }
+
+    /// <summary>Max Warmth decay multiplier from incubated stages (1 when healthy).</summary>
+    public static float WarmthDecayFactor(Person person, DefCatalog catalog, DateTime now)
+    {
+        ArgumentNullException.ThrowIfNull(person);
+        ArgumentNullException.ThrowIfNull(catalog);
+        var list = person.Conditions;
+        if (list is null || list.Count == 0)
+        {
+            return 1f;
+        }
+
+        var factor = 1f;
+        foreach (var condition in list)
+        {
+            if (!catalog.Diseases.TryGetValue(condition.DefName, out var disease) || disease.Abstract)
+            {
+                continue;
+            }
+
+            if (!IsIncubated(condition, disease, now) || !TryStage(disease, condition.Severity, out var stage))
+            {
+                continue;
+            }
+
+            if (stage.WarmthDecayFactor > factor)
+            {
+                factor = stage.WarmthDecayFactor;
+            }
+        }
+
+        return factor;
+    }
+}