Added the system from trail

Runtime/SubclassSelectorAttribute.cs

@@ -0,0 +1,13 @@
+using System;
+using UnityEngine;
+
+namespace Jovian.EncounterSystem {
+    /// <summary>
+    /// Marker attribute instructing the editor to render a concrete-type picker for a
+    /// <c>[SerializeReference]</c> field (or list/array element). The drawer scans the declared
+    /// base type's assembly for non-abstract implementations and offers them in a dropdown.
+    /// </summary>
+    [AttributeUsage(AttributeTargets.Field, AllowMultiple = false)]
+    public class SubclassSelectorAttribute : PropertyAttribute {
+    }
+}