Add person-to-person disease contagion with outbreak notices.

Node/class contact rolls use pair+day seeds; vanilla respiratory diseases spread, and a threshold raises an info toast instead of per-sneeze noise.

Co-authored-by: Cursor <cursoragent@cursor.com>

src/HSchool.Content/DiseaseDefValidator.cs

@@ -46,11 +46,19 @@ internal static class DiseaseDefValidator
             if (def.BaseChancePerDay < 0f
                 || def.ColdChancePerDay < 0f
                 || def.RainChancePerDay < 0f
-                || def.SnowChancePerDay < 0f)
+                || def.SnowChancePerDay < 0f
+                || def.NodeContagionChancePerDay < 0f
+                || def.ClassContagionChancePerDay < 0f)
             {
                 throw new ContentLoadException($"DiseaseDef '{def.DefName}' chance fields cannot be negative.");
             }
 
+            if (def.NodeContagionChancePerDay > 1f || def.ClassContagionChancePerDay > 1f)
+            {
+                throw new ContentLoadException(
+                    $"DiseaseDef '{def.DefName}' contagion chances must be 0–1.");
+            }
+
             DiseaseStage? previous = null;
             foreach (var stage in def.Stages)
             {

src/HSchool.Content/DiseaseDefs.cs

@@ -31,8 +31,8 @@ public sealed class DiseaseStage
 }
 
 /// <summary>
-/// A named disease: stages, weather vectors, lesson/attendance effects, temporary immunity.
-/// Contagion between people is phase 79 — these defs may omit it.
+/// A named disease: stages, weather vectors, lesson/attendance effects, temporary immunity,
+/// and optional person-to-person contagion (node and/or class).
 /// </summary>
 public sealed class DiseaseDef : Def
 {
@@ -58,4 +58,18 @@ public sealed class DiseaseDef : Def
 
     /// <summary>Days of immunity to this def after recovery.</summary>
     public float ImmunityDays { get; init; }
+
+    /// <summary>
+    /// Daily chance to infect each other person sharing the same map node. 0 — no node contagion.
+    /// </summary>
+    public float NodeContagionChancePerDay { get; init; }
+
+    /// <summary>
+    /// Daily chance to infect each classmate (same <c>ClassId</c>) while both are on campus.
+    /// 0 — no class contagion.
+    /// </summary>
+    public float ClassContagionChancePerDay { get; init; }
+
+    /// <summary>When true, the carrier can infect during incubation before stage effects apply.</summary>
+    public bool ContagiousDuringIncubation { get; init; }
 }

src/HSchool.Content/EventDefValidator.cs

@@ -16,6 +16,7 @@ internal static class EventDefValidator
         EventTriggers.GenerationFailed,
         EventTriggers.DirectorSummon,
         EventTriggers.ParentMeeting,
+        EventTriggers.DiseaseOutbreak,
     };
 
     private static readonly HashSet<string> Actions = new(StringComparer.Ordinal)

src/HSchool.Content/EventDefs.cs

@@ -14,6 +14,7 @@ public static class EventTriggers
     public const string GenerationFailed = "generationFailed";
     public const string DirectorSummon = "directorSummon";
     public const string ParentMeeting = "parentMeeting";
+    public const string DiseaseOutbreak = "diseaseOutbreak";
 }
 
 public static class EventActions

src/HSchool.Content/PeopleDefValidator.cs

@@ -757,6 +757,12 @@ internal static class PeopleDefValidator
             throw new ContentLoadException(
                 $"BehaviorDef '{behavior.DefName}' diseaseVectorScale cannot be negative.");
         }
+
+        if (behavior.DiseaseOutbreakMinNewCases < 0)
+        {
+            throw new ContentLoadException(
+                $"BehaviorDef '{behavior.DefName}' diseaseOutbreakMinNewCases cannot be negative.");
+        }
     }
 
     private static void ValidateTopic(TopicDef topic, DefCatalog catalog)

src/HSchool.Content/PeopleDefs.cs

@@ -660,6 +660,12 @@ public sealed class BehaviorDef : Def
     /// </summary>
     public float DiseaseVectorScale { get; init; } = 1f;
 
+    /// <summary>
+    /// New contagion cases in one day at or above this raise a diseaseOutbreak world event.
+    /// 0 disables the toast.
+    /// </summary>
+    public int DiseaseOutbreakMinNewCases { get; init; } = 3;
+
     public static IReadOnlyList<float> DefaultLessonMarkThresholds { get; } =
     [
         0.85f,