Added the system from trail

Runtime/IRewardKind.cs

@@ -0,0 +1,46 @@
+using System;
+
+namespace Jovian.EncounterSystem {
+    /// <summary>
+    /// Marker interface for the polymorphic payload of a <see cref="Reward"/>.
+    /// Each concrete kind carries its own designer-facing data. Behaviour lives in a reward-applying
+    /// handler registered on <see cref="EncounterResolver"/> for <see cref="GiveRewardEvent"/> —
+    /// kinds are pure data. Add a new reward type by creating a new <see cref="IRewardKind"/>
+    /// implementation; the SubclassSelector drawer will surface it automatically.
+    /// </summary>
+    public interface IRewardKind {
+    }
+
+    /// <summary>Grants an amount of a named currency (gold, silver, faction-specific scrip, ...).</summary>
+    [Serializable]
+    public class CurrencyRewardKind : IRewardKind {
+        public string currencyId;
+        public int amount;
+    }
+
+    /// <summary>Grants one or more copies of an item identified by id.</summary>
+    [Serializable]
+    public class ItemRewardKind : IRewardKind {
+        public string itemId;
+        public int quantity;
+    }
+
+    /// <summary>Grants experience points to the party or a specific recipient.</summary>
+    [Serializable]
+    public class ExperienceRewardKind : IRewardKind {
+        public int amount;
+    }
+
+    /// <summary>Unlocks something identified by id (recipe, area, journal entry, achievement, ...).
+    /// What is actually unlocked is decided by the game-side handler.</summary>
+    [Serializable]
+    public class UnlockableRewardKind : IRewardKind {
+        public string unlockableId;
+    }
+
+    /// <summary>Catch-all with no kind-specific data — useful for prototyping or one-off rewards
+    /// whose semantics are carried by the <see cref="Reward.id"/> alone.</summary>
+    [Serializable]
+    public class OtherRewardKind : IRewardKind {
+    }
+}