new builder option per draft - first one is gold per kill

Assets/_Project/Scripts/Core/BuilderEffectKind.cs

@@ -0,0 +1,14 @@
+// Assets/_Project/Scripts/Core/BuilderEffectKind.cs
+namespace TD.Core
+{
+    /// <summary>
+    /// Identifies which builder effect a
+    /// <see cref="TD.Gameplay.BuilderEffects.BuilderEffectDefinition"/> represents. Backs a
+    /// bitmask on <see cref="TD.Gameplay.BuilderUpgradeManager"/>, so keep this to 32 entries
+    /// or fewer (or widen the mask to ulong if it ever needs more).
+    /// </summary>
+    public enum BuilderEffectKind : byte
+    {
+        GoldPerKill = 0,
+    }
+}