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/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; }
 }