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/Abilities/MonsterAbility.cs

@@ -1,5 +1,4 @@
 using System;
-using System.Collections.Generic;
 using ModernUO.CodeGeneratedEvents;
 
 namespace Server.Mobiles;
@@ -9,8 +8,6 @@ namespace Server.Mobiles;
 /// </summary>
 public abstract class MonsterAbility
 {
-    private Dictionary<BaseCreature, long> _nextTriggerTicks;
-
     public virtual MonsterAbilityType AbilityType => MonsterAbilityType.Generic;
 
     public virtual MonsterAbilityTrigger AbilityTrigger => MonsterAbilityTrigger.None;
@@ -30,18 +27,9 @@ public abstract class MonsterAbility
             return false;
         }
 
-        if (_nextTriggerTicks?.TryGetValue(source, out var nextTrigger) == true)
+        if (source.IsAbilityOnCooldown(this))
         {
-            if (nextTrigger - Core.TickCount > 0)
-            {
-                return false;
-            }
-
-            _nextTriggerTicks.Remove(source);
-            if (_nextTriggerTicks.Count == 0)
-            {
-                _nextTriggerTicks = null;
-            }
+            return false;
         }
 
         var c = ChanceToTrigger;
@@ -65,10 +53,7 @@ public abstract class MonsterAbility
             ? MinTriggerCooldown
             : Utility.RandomMinMax(MinTriggerCooldown, MaxTriggerCooldown);
 
-        var nextTrigger = Core.TickCount + (long)randomAmount.TotalMilliseconds;
-
-        _nextTriggerTicks ??= new Dictionary<BaseCreature, long>();
-        _nextTriggerTicks[source] = nextTrigger;
+        source.SetAbilityCooldown(this, randomAmount);
     }
 
     public virtual void AlterMeleeDamageFrom(BaseCreature source, Mobile target, ref int damage)
@@ -100,18 +85,5 @@ public abstract class MonsterAbility
     }
 
     [OnEvent(nameof(CreatureEvents.CreatureDeathEvent))]
-    [OnEvent(nameof(CreatureEvents.CreatureDeletedEvent))]
-    public static void InvalidateNextAbilityTriggers(BaseCreature source)
-    {
-        var abilities = source.GetMonsterAbilities();
-        if (abilities == null || abilities.Length == 0)
-        {
-            return;
-        }
-
-        for (var i = 0; i < abilities.Length; i++)
-        {
-            abilities[i]._nextTriggerTicks?.Remove(source);
-        }
-    }
+    public static void ResetCooldownsOnDeath(BaseCreature source) => source.ClearAbilityCooldowns();
 }