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.Content/DiseaseDefs.cs

@@ -0,0 +1,61 @@
+namespace HSchool.Content;
+
+/// <summary>Stable family ids for immunity grouping and realism labels.</summary>
+public static class DiseaseFamilies
+{
+    public const string Respiratory = "respiratory";
+
+    public const string Gastrointestinal = "gastrointestinal";
+
+    public const string Ent = "ent";
+}
+
+/// <summary>One severity band on a <see cref="DiseaseDef"/> curve.</summary>
+public sealed class DiseaseStage
+{
+    /// <summary>Inclusive lower bound of severity for this stage.</summary>
+    public float MinSeverity { get; init; }
+
+    public float SeverityPerDay { get; init; }
+
+    public float ProgressPerDay { get; init; }
+
+    /// <summary>Multiplies lesson skill gain while this stage is active (after incubation).</summary>
+    public float LessonLearningFactor { get; init; } = 1f;
+
+    /// <summary>Per-day chance the person stays off campus while in this stage (after incubation).</summary>
+    public float StayHomeChance { get; init; }
+
+    /// <summary>Multiplies Warmth decay. 1 = unchanged.</summary>
+    public float WarmthDecayFactor { get; init; } = 1f;
+}
+
+/// <summary>
+/// A named disease: stages, weather vectors, lesson/attendance effects, temporary immunity.
+/// Contagion between people is phase 79 — these defs may omit it.
+/// </summary>
+public sealed class DiseaseDef : Def
+{
+    /// <summary><see cref="DiseaseFamilies"/> id. Required on concrete defs.</summary>
+    public string Family { get; init; } = "";
+
+    /// <summary>Game days after onset before stage effects (lesson / stay-home) apply.</summary>
+    public float IncubationDays { get; init; }
+
+    public IReadOnlyList<DiseaseStage> Stages { get; init; } = [];
+
+    /// <summary>Base daily onset chance before weather vectors and <see cref="BehaviorDef.DiseaseVectorScale"/>.</summary>
+    public float BaseChancePerDay { get; init; }
+
+    /// <summary>Street °C at or below this adds <see cref="ColdChancePerDay"/>. Null — no cold vector.</summary>
+    public float? ColdBelowC { get; init; }
+
+    public float ColdChancePerDay { get; init; }
+
+    public float RainChancePerDay { get; init; }
+
+    public float SnowChancePerDay { get; init; }
+
+    /// <summary>Days of immunity to this def after recovery.</summary>
+    public float ImmunityDays { get; init; }
+}