first pass at enemy upgrades

Assets/_Project/Scripts/Core/EnemyAbilityKind.cs

@@ -0,0 +1,16 @@
+namespace TD.Core
+{
+    /// <summary>
+    /// Identifies which enemy ability a
+    /// <see cref="TD.Gameplay.EnemyAbilities.EnemyAbilityDefinition"/> represents. Mirrors
+    /// <see cref="BuilderSpellKind"/> — a stable, enum-indexed identifier used by
+    /// <see cref="TD.Gameplay.EnemyAbilities.EnemyAbilityPool"/> instead of an asset reference.
+    /// </summary>
+    public enum EnemyAbilityKind : byte
+    {
+        /// <summary>No ability rolled. Never a real pool entry — only the sentinel value
+        /// <see cref="TD.Gameplay.EnemyAbility"/> reports before/absent a roll.</summary>
+        None = 0,
+        SplitOnDeath = 1,
+    }
+}

Assets/_Project/Scripts/Core/EnemyAbilityKind.cs.meta

@@ -0,0 +1,2 @@
+fileFormatVersion: 2
+guid: ec52a6b404fed65dfa48a63b5535373d