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

Assets/_Project/Scripts/Gameplay/BuilderEffects/GoldPerKillEffectDefinition.cs

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+// Assets/_Project/Scripts/Gameplay/BuilderEffects/GoldPerKillEffectDefinition.cs
+using UnityEngine;
+using TD.Core;
+
+namespace TD.Gameplay.BuilderEffects
+{
+    /// <summary>
+    /// Builder effect #1 — every tower the player owns yields extra gold per kill.
+    /// Read by <see cref="BuilderUpgradeManager.GetTotalGoldPerKillBonus"/>, which
+    /// <see cref="TD.Gameplay.WaveManager"/> queries when awarding kill gold.
+    /// </summary>
+    [CreateAssetMenu(fileName = "GoldPerKillEffect", menuName = "TD/Builder Effects/Gold Per Kill")]
+    public class GoldPerKillEffectDefinition : BuilderEffectDefinition
+    {
+        public override BuilderEffectKind Kind => BuilderEffectKind.GoldPerKill;
+
+        [Tooltip("Flat bonus gold awarded on top of the normal kill reward, for every kill by " +
+                 "any of this player's towers.")]
+        [Min(0)]
+        public int BonusGoldPerKill = 1;
+    }
+}