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.

tests/HSchool.People.Tests/DiseaseEffectTests.cs

@@ -0,0 +1,161 @@
+using HSchool.Content;
+
+namespace HSchool.People.Tests;
+
+public class DiseaseEffectTests
+{
+    [Fact]
+    public void TwoDiseaseDefs_YieldDifferentSeverityCurves()
+    {
+        var catalog = LoadVanilla();
+        var cold = catalog.Diseases["CommonCold"];
+        var flu = catalog.Diseases["Influenza"];
+        var started = new DateTime(2012, 4, 3, 8, 0, 0, DateTimeKind.Utc);
+        var a = Blank("p1");
+        var b = Blank("p1");
+        HealthConditions.Add(a, Onset(cold.DefName, started, severity: 0.1f));
+        HealthConditions.Add(b, Onset(flu.DefName, started, severity: 0.1f));
+
+        Assert.True(HealthConditions.Tick(a, peopleSeed: 7, dayNumber: 100, gameMinutes: 24 * 60, catalog, started));
+        Assert.True(HealthConditions.Tick(b, peopleSeed: 7, dayNumber: 100, gameMinutes: 24 * 60, catalog, started));
+
+        Assert.NotEqual(a.Conditions![0].Severity, b.Conditions![0].Severity);
+        Assert.True(a.Conditions[0].Severity > 0.1f);
+        Assert.True(b.Conditions[0].Severity > 0.1f);
+    }
+
+    [Fact]
+    public void HeavyStage_StaysHomeMoreOftenThanMild()
+    {
+        var catalog = LoadVanilla();
+        var started = new DateTime(2012, 1, 1, 6, 0, 0, DateTimeKind.Utc);
+        var mild = Blank("pupil");
+        var heavy = Blank("pupil");
+        HealthConditions.Add(mild, Onset("Influenza", started, severity: 0.1f));
+        HealthConditions.Add(heavy, Onset("Influenza", started, severity: 0.8f));
+
+        // Past incubation so stage effects apply. Same person, different days — the day salt varies the roll.
+        var day = started.AddDays(2);
+        var mildHome = 0;
+        var heavyHome = 0;
+        for (var i = 0; i < 200; i++)
+        {
+            var when = day.AddDays(i);
+            if (DiseaseEffects.ShouldStayHome(mild, catalog, peopleSeed: 11, when))
+            {
+                mildHome++;
+            }
+
+            if (DiseaseEffects.ShouldStayHome(heavy, catalog, peopleSeed: 11, when))
+            {
+                heavyHome++;
+            }
+        }
+
+        Assert.True(heavyHome > mildHome);
+        Assert.True(heavyHome > 100);
+    }
+
+    [Fact]
+    public void Attendance_IllnessReason_IsNotTruancy()
+    {
+        var person = Blank("p1");
+        var rules = new BehaviorDef { DefName = "Behavior", AttendanceMax = 10 };
+        var time = new DateTime(2012, 4, 3, 9, 15, 0, DateTimeKind.Utc);
+        Assert.True(AttendanceMemory.Record(
+            person,
+            "Mathematics",
+            AttendanceStatuses.Absent,
+            time,
+            period: 1,
+            rules,
+            AbsenceReasons.Illness));
+
+        Assert.Equal(AbsenceReasons.Illness, person.Attendance![0].AbsenceReason);
+        Assert.NotEqual(AbsenceReasons.Truancy, person.Attendance[0].AbsenceReason);
+
+        var json = RosterJson.Serialize(RosterDocument.From(1, new Roster([person], [], [])));
+        var loaded = RosterJson.Parse(json).ToRoster().People[0];
+        Assert.Equal(AbsenceReasons.Illness, loaded.Attendance![0].AbsenceReason);
+    }
+
+    [Fact]
+    public void Recovery_GrantsTemporaryImmunity()
+    {
+        var catalog = LoadVanilla();
+        var started = new DateTime(2012, 4, 3, 8, 0, 0, DateTimeKind.Utc);
+        var person = Blank("p1");
+        HealthConditions.Add(
+            person,
+            new HealthCondition
+            {
+                DefName = "CommonCold",
+                Severity = 0.2f,
+                Progress = 0.99f,
+                Source = "cold",
+                StartedAt = started,
+            });
+
+        Assert.True(HealthConditions.Tick(
+            person,
+            peopleSeed: 3,
+            dayNumber: 50,
+            gameMinutes: 24 * 60 * 3,
+            catalog,
+            started.AddDays(1)));
+
+        Assert.Null(person.Conditions);
+        Assert.NotNull(person.DiseaseImmunities);
+        Assert.Equal("CommonCold", person.DiseaseImmunities![0].DefName);
+        Assert.True(person.DiseaseImmunities[0].Until > started);
+        Assert.True(DiseaseImmunities.IsImmune(person, "CommonCold", started.AddDays(2)));
+    }
+
+    private static HealthCondition Onset(string defName, DateTime started, float severity) =>
+        new()
+        {
+            DefName = defName,
+            Severity = severity,
+            Progress = 0f,
+            Source = "cold",
+            StartedAt = DateTime.SpecifyKind(started, DateTimeKind.Utc),
+        };
+
+    private static DefCatalog LoadVanilla()
+    {
+        var root = Path.Combine(AppContext.BaseDirectory, "vanilla");
+        return new CatalogLoader().Load(
+            [CatalogLoader.CorePackId],
+            PackDocuments.FromDirectory(CatalogLoader.CorePackId, root));
+    }
+
+    private static Person Blank(string id)
+    {
+        var cases = new CaseTable
+        {
+            Nom = id,
+            Gen = id,
+            Dat = id,
+            Acc = id,
+            Ins = id,
+            Pre = id,
+        };
+        return new Person
+        {
+            Id = id,
+            FamilyId = "f",
+            Female = false,
+            BirthDate = new DateTime(2000, 1, 1, 0, 0, 0, DateTimeKind.Utc),
+            Name = new PersonName(id, id, id, cases, cases, cases),
+            IsStudent = true,
+            IsStaff = false,
+            IsParent = false,
+            Numbers = new Dictionary<string, int>(StringComparer.Ordinal),
+            Choices = new Dictionary<string, string>(StringComparer.Ordinal),
+            Skills = new Dictionary<string, int>(StringComparer.Ordinal),
+            Traits = [],
+            Needs = new Dictionary<string, float>(StringComparer.Ordinal),
+            Opinions = new Dictionary<string, int>(StringComparer.Ordinal),
+        };
+    }
+}