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

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

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+// Assets/_Project/Scripts/Gameplay/BuilderEffects/BuilderEffectDefinition.cs
+using UnityEngine;
+using TD.Core;
+
+namespace TD.Gameplay.BuilderEffects
+{
+    /// <summary>
+    /// Base class for one builder effect — a passive perk that applies to every tower a
+    /// player's Builder constructs (e.g. "kills yield bonus gold"). Granted via the draft
+    /// (<see cref="TD.Gameplay.Draft.BuilderEffectDraftOption"/>) and tracked per-player by
+    /// <see cref="BuilderUpgradeManager"/>.
+    /// </summary>
+    /// <remarks>
+    /// <para><b>One asset per kind.</b> <see cref="Kind"/> is fixed per subclass (each concrete
+    /// type overrides it to its own <see cref="BuilderEffectKind"/>), so there's no way to
+    /// mis-tag an asset in the inspector. <see cref="BuilderEffectPool"/> uses it to build a
+    /// fixed-size, enum-indexed lookup table — no scanning to resolve a kind to its data.</para>
+    ///
+    /// <para><b>No shared "Apply" contract.</b> Concrete subclasses (e.g.
+    /// <see cref="GoldPerKillEffectDefinition"/>) carry whatever payload they need. Effects with
+    /// different application shapes — a simple value read at an existing game-event hook vs.
+    /// behavior that needs its own component on the tower — are still just subclasses; consumers
+    /// query the concrete type they care about directly (see
+    /// <see cref="BuilderUpgradeManager.GetTotalGoldPerKillBonus"/>).</para>
+    /// </remarks>
+    public abstract class BuilderEffectDefinition : ScriptableObject
+    {
+        /// <summary>Which builder effect this asset's data belongs to.</summary>
+        public abstract BuilderEffectKind Kind { get; }
+
+        [Tooltip("Name shown wherever a player's active builder effects are listed.")]
+        public string DisplayName;
+    }
+}