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;
+    }
+}

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

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+fileFormatVersion: 2
+guid: 3a939cb6af19202e1a9bddf01d9f1857

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

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+// Assets/_Project/Scripts/Gameplay/BuilderEffects/BuilderEffectPool.cs
+using System;
+using UnityEngine;
+using TD.Core;
+
+namespace TD.Gameplay.BuilderEffects
+{
+    /// <summary>
+    /// Scene singleton holding every <see cref="BuilderEffectDefinition"/> available this match,
+    /// authored as a flat array in the inspector for convenience. Internally it builds a
+    /// fixed-size table indexed by <see cref="BuilderEffectKind"/> so lookups are a single array
+    /// access, not a scan.
+    /// </summary>
+    /// <remarks>
+    /// Plain MonoBehaviour: identical on every peer (same assets), so there is nothing to sync.
+    /// The server reads it to resolve grants; clients read it to render an "active effects" list.
+    /// </remarks>
+    public class BuilderEffectPool : MonoBehaviour
+    {
+        public static BuilderEffectPool Instance { get; private set; }
+
+        [Tooltip("Every BuilderEffectDefinition asset available this match. One entry per " +
+                 "BuilderEffectKind — order doesn't matter, Kind on the asset itself decides " +
+                 "its slot.")]
+        [SerializeField] private BuilderEffectDefinition[] effects;
+
+        // Fixed-size, enum-indexed lookup built once in Awake. Sized to the enum's entry count,
+        // not authored count, so an out-of-range Kind is a compile-time impossibility rather
+        // than a bounds check we'd otherwise need on every Get().
+        private BuilderEffectDefinition[] byKind;
+
+        private void Awake()
+        {
+            if (Instance != null && Instance != this)
+            {
+                Debug.LogError("[BuilderEffectPool] Multiple instances detected. Only one per scene.");
+                return;
+            }
+            Instance = this;
+
+            int kindCount = Enum.GetValues(typeof(BuilderEffectKind)).Length;
+            byKind = new BuilderEffectDefinition[kindCount];
+            if (effects == null) return;
+
+            for (int i = 0; i < effects.Length; i++)
+            {
+                var def = effects[i];
+                if (def == null) continue;
+                byKind[(int)def.Kind] = def;
+            }
+        }
+
+        private void OnDestroy()
+        {
+            if (Instance == this) Instance = null;
+        }
+
+        /// <summary>Returns the effect asset for <paramref name="kind"/>, or null if none is
+        /// authored in this pool.</summary>
+        public BuilderEffectDefinition Get(BuilderEffectKind kind)
+        {
+            int i = (int)kind;
+            return (byKind != null && i >= 0 && i < byKind.Length) ? byKind[i] : null;
+        }
+    }
+}

Assets/_Project/Scripts/Gameplay/BuilderEffects/BuilderEffectPool.cs.meta

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+fileFormatVersion: 2
+guid: 6226c6e672c5b358a9cdc2a1bee47daf

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;
+    }
+}

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

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+fileFormatVersion: 2
+guid: 5ce77a7f27ed228a0ad0494dfc1a4749