fix: monster ability queries no longer have side effects (#2716)

## Problem
`GetMonsterAbilities()` answers three questions through one virtual: which abilities a creature has (`GetAbility`, `AOS.cs` reflect/barrier, bard fire-breath difficulty), which abilities are enabled right now, and where cooldown state must be cleaned up (`InvalidateNextAbilityTriggers` on death and delete). Every caller assumes it is a pure, stable list. About 70 creatures return a static array; only LordOaks computed it:

```
Mobile.Delete()
 → BaseCreature.OnDelete() → CreatureDeletedEvent
 → MonsterAbility.InvalidateNextAbilityTriggers()
 → LordOaks.GetMonsterAbilities() → CheckQueen() → new Silvani().MoveToWorld(...)
```

- Deleting a Lord Oaks nobody had fought (champion reset, `DeleteCreatures`, GM delete) spawned an orphaned Silvani where he stood.
- The spell damage scalar (`AlterDamageScalarFrom`) spawned her too.
- With the queen dead his list was empty, so cleanup skipped his cooldown entries. The static `Dictionary<BaseCreature, long>` on the ability singleton kept a reference to the deleted creature. `MonsterAbilityGroup` children's entries were never cleaned either.
- Neira threw unholy bones from `AlterDamageScalarFrom`, so any caller asking for a damage scalar (a preview gump, for example) could fire the effect.

## Changes
- **Per-creature cooldowns:** a lazy `Dictionary<MonsterAbility, long>` on `BaseCreature` (`IsAbilityOnCooldown`, `SetAbilityCooldown`, `ClearAbilityCooldowns`). The static dictionaries and the query-driven cleanup are gone, and death still resets cooldowns. Tick comparisons are wraparound-safe.
- **Pure contract:** `GetMonsterAbilities()` is documented as pure. The new `virtual bool CanUseAbility(MonsterAbility)` gates abilities that only work some of the time. Every `TriggerAbility*` loop and `GetAbility` check it.
- **LordOaks:** always lists his abilities; `CanUseAbility` and the 10% spell shield read "queen alive". The queen spawns when he is first aggressed or first given a combatant, never on the internal map. Aggression lands before the damage it causes, so the first spell is still shielded, as before.
- **Neira:** the unholy bone throw moves to `OnDamagedBySpell`.

A follow-up PR makes minions such as the queen, pixies and Barracoon's rats belong to their champion spawn, so a spawn reset cleans them up.

## Testing
- `LordOaksTests` covers three cases:
  - querying or deleting an unengaged Lord Oaks spawns nothing;
  - aggression spawns the queen exactly once, and the shield and abilities follow her life;
  - an Oaks on the internal map never spawns her.
- UOContent.Tests: 1234 passed, 2 skipped.
- A local constructor sweep (every Mobile built and deleted repeatedly) no longer reports LordOaks.

Projects/UOContent/Mobiles/BaseCreature.cs

@@ -1,4 +1,4 @@
-using System;
+using System;
 using System.Collections.Generic;
 using System.Runtime.CompilerServices;
 using ModernUO.Serialization;
@@ -1500,10 +1500,47 @@ namespace Server.Mobiles
         public List<MLQuest> MLQuests =>
             (m_MLQuests ??= StaticMLQuester ? MLQuestSystem.FindQuestList(GetType()) : ConstructQuestList()) ?? MLQuestSystem.EmptyList;
 
+        /// <summary>
+        ///     The abilities this creature has. Must be a pure, stable query: it is called from lookups, damage
+        ///     calculations and cleanup, so it must not spawn, mutate state, or change with the creature's state.
+        ///     Gate abilities that should only work some of the time with <see cref="CanUseAbility" />.
+        /// </summary>
         public virtual MonsterAbility[] GetMonsterAbilities() => null;
 
+        /// <summary>
+        ///     Whether <paramref name="ability" /> may trigger or be looked up right now. Must be a pure read.
+        /// </summary>
+        public virtual bool CanUseAbility(MonsterAbility ability) => true;
+
         private MonsterAbilityTrigger _activeTriggers;
 
+        // Deadlines in Core.TickCount, keyed by ability. Lives on the creature so it is collected with it.
+        private Dictionary<MonsterAbility, long> _abilityCooldowns;
+
+        public bool IsAbilityOnCooldown(MonsterAbility ability)
+        {
+            if (_abilityCooldowns?.TryGetValue(ability, out var nextTrigger) != true)
+            {
+                return false;
+            }
+
+            if (nextTrigger - Core.TickCount > 0)
+            {
+                return true;
+            }
+
+            _abilityCooldowns.Remove(ability);
+            return false;
+        }
+
+        public void SetAbilityCooldown(MonsterAbility ability, TimeSpan cooldown)
+        {
+            _abilityCooldowns ??= new Dictionary<MonsterAbility, long>();
+            _abilityCooldowns[ability] = Core.TickCount + (long)cooldown.TotalMilliseconds;
+        }
+
+        public void ClearAbilityCooldowns() => _abilityCooldowns = null;
+
         public virtual MonsterAbility GetAbility(MonsterAbilityType type)
         {
             var abilities = GetMonsterAbilities();
@@ -1516,6 +1553,11 @@ namespace Server.Mobiles
             for (var i = 0; i < abilities.Length; i++)
             {
                 var ability = abilities[i];
+                if (!CanUseAbility(ability))
+                {
+                    continue;
+                }
+
                 if (ability is MonsterAbilityGroup group)
                 {
                     ability = group.GetAbilityWithType(type);
@@ -1579,7 +1621,7 @@ namespace Server.Mobiles
                 for (var i = 0; i < abilities.Length; i++)
                 {
                     var ability = abilities[i];
-                    if (ability.CanTrigger(this, trigger))
+                    if (CanUseAbility(ability) && ability.CanTrigger(this, trigger))
                     {
                         ability.Trigger(trigger, this, defender);
                         triggered = true;
@@ -1606,7 +1648,7 @@ namespace Server.Mobiles
             for (var i = 0; i < abilities.Length; i++)
             {
                 var ability = abilities[i];
-                if (ability.CanTrigger(this, trigger))
+                if (CanUseAbility(ability) && ability.CanTrigger(this, trigger))
                 {
                     ability.Move(this, d);
                 }
@@ -1625,7 +1667,7 @@ namespace Server.Mobiles
             for (var i = 0; i < abilities.Length; i++)
             {
                 var ability = abilities[i];
-                if (ability.CanTrigger(this, trigger))
+                if (CanUseAbility(ability) && ability.CanTrigger(this, trigger))
                 {
                     if ((trigger & MonsterAbilityTrigger.GiveMeleeDamage) != 0)
                     {
@@ -1666,7 +1708,7 @@ namespace Server.Mobiles
             for (var i = 0; i < abilities.Length; i++)
             {
                 var ability = abilities[i];
-                if (ability.CanTrigger(this, trigger))
+                if (CanUseAbility(ability) && ability.CanTrigger(this, trigger))
                 {
                     if ((trigger & MonsterAbilityTrigger.GiveSpellDamage) != 0)
                     {