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.People/DiseaseEffects.cs

@@ -4,7 +4,7 @@ namespace HSchool.People;
 
 /// <summary>
 /// Reads DiseaseDef stages for lesson gain, stay-home rolls and warmth decay.
-/// Contagion between people is phase 79.
+/// Contagion rolls live in DiseaseSystem; this type only answers stage questions.
 /// </summary>
 public static class DiseaseEffects
 {

src/HSchool.People/Seed.cs

@@ -26,6 +26,7 @@ public static class Seed
     public const int MeetingSalt = 18;
     public const int MeetingAttendSalt = 19;
     public const int DiseaseSalt = 20;
+    public const int ContagionSalt = 21;
 
     /// <summary>A stream that belongs to the school rather than to one family.</summary>
     public static int ForSchool(int schoolSeed, int salt) => Mix(schoolSeed, familyIndex: -1, salt);
@@ -49,6 +50,19 @@ public static class Seed
         return (int)z;
     }
 
+    /// <summary>
+    /// Contact roll for an ordered person pair on one day (contagion). Ids are sorted so
+    /// A→B and B→A share the stream; channel salt separates node vs class.
+    /// </summary>
+    public static int MixPair(int schoolSeed, string personA, string personB, int dayNumber, int salt)
+    {
+        ArgumentNullException.ThrowIfNull(personA);
+        ArgumentNullException.ThrowIfNull(personB);
+        var first = string.CompareOrdinal(personA, personB) <= 0 ? personA : personB;
+        var second = string.CompareOrdinal(personA, personB) <= 0 ? personB : personA;
+        return Mix(schoolSeed, first + "\u001f" + second, dayNumber, salt);
+    }
+
     private static uint Stable(string value)
     {
         ulong z = 0;