first pass at enemy upgrades

Assets/_Project/Scripts/Gameplay/EnemyAbilities/EnemyAbilityDefinition.cs

@@ -0,0 +1,55 @@
+// Assets/_Project/Scripts/Gameplay/EnemyAbilities/EnemyAbilityDefinition.cs
+using UnityEngine;
+using TD.Core;
+
+namespace TD.Gameplay.EnemyAbilities
+{
+    /// <summary>
+    /// Base class for one enemy ability (e.g. "split into smaller enemies on death"). Rolled
+    /// at random for each spawned enemy by <see cref="EnemyAbilityPool.RollRandom"/> and applied
+    /// via <see cref="EnemyAbility.InitializeServer"/>.
+    /// </summary>
+    /// <remarks>
+    /// <para><b>One asset per kind.</b> <see cref="Kind"/> is fixed per subclass, same as
+    /// <see cref="BuilderSpells.BuilderSpellDefinition"/>. <see cref="EnemyAbilityPool"/> uses
+    /// it to build a fixed-size, enum-indexed lookup table.</para>
+    ///
+    /// <para><b>Server-only hooks.</b> All three hooks below only ever run on the server —
+    /// <see cref="EnemyAbility"/> only calls them when <c>IsServer</c> is true. <see
+    /// cref="ServerOnSpawn"/> and <see cref="ServerTick"/> are no-ops for abilities that don't
+    /// need spawn-time setup or a per-frame timer (e.g. Split on Death uses neither); they exist
+    /// so a future cooldown-driven ability (disable towers, teleport) doesn't need a base-class
+    /// change.</para>
+    /// </remarks>
+    public abstract class EnemyAbilityDefinition : ScriptableObject
+    {
+        /// <summary>Which enemy ability this asset's data belongs to.</summary>
+        public abstract EnemyAbilityKind Kind { get; }
+
+        [Header("Presentation")]
+        [Tooltip("Name shown in debug logs and future enemy-info UI.")]
+        public string DisplayName;
+
+        [Tooltip("Short description shown in future enemy-info UI.")]
+        [TextArea(2, 4)]
+        public string Description;
+
+        [Header("Selection")]
+        [Tooltip("Relative weight of this ability being rolled, vs. the pool's other abilities " +
+                 "and its no-ability chance. Same semantics as DraftOption.Weight.")]
+        [Min(0f)]
+        public float Weight = 1f;
+
+        /// <summary>Server-only: called once, right after this ability is assigned (before the
+        /// enemy's NetworkObject is spawned to clients). Default no-op.</summary>
+        public virtual void ServerOnSpawn(EnemyAbility instance) { }
+
+        /// <summary>Server-only: called every frame this ability is active on a live enemy.
+        /// Default no-op.</summary>
+        public virtual void ServerTick(EnemyAbility instance, float dt) { }
+
+        /// <summary>Server-only: called the instant the enemy's HP reaches zero, before the
+        /// death animation/despawn sequence plays. Default no-op.</summary>
+        public virtual void ServerOnDeath(EnemyAbility instance, EnemyHealth health) { }
+    }
+}