feat: Adds new decay system and SkipSerialization (#2311)

## Summary

- Replaces scan-based decay checking during world saves with an event-driven timer wheel scheduler
- Removes virtual property checks from serialization hot path, achieving 6-8x faster world saves

## Changes

### DecayScheduler (new):
- Timer wheel with 12 HashSet buckets (5-min intervals) + PriorityQueue for active processing
- O(1) register/unregister vs O(n) scan of all items
- Auto start/stop when items exist/empty
- Configurable tick interval with jitter to prevent system synchronization

### Item.cs:
- Added ScheduledDecayTime computed property
- Added UpdateDecayRegistration() called from SetLastMoved(), property setters, AddItem()/RemoveItem()
- Hooks in Visible, Movable, Spawner setters and Delete()

### World.cs:
- Removed _decayQueue, EnqueueForDecay(), ProcessDecay()
- ItemPersistence.Serialize() now tight loop without virtual calls

## Performance

| Metric        | Before     | After     | Improvement |
|---------------|------------|-----------|-------------|
| Items (230K)  | 245K ticks | 34K ticks | 8x faster   |
| Mobiles (43K) | 56K ticks  | 6K ticks  | 9x faster   |
| Total         | 302K ticks | 40K ticks | 7.5x faster |

## Configuration

decay.maxItemsPerTick = 250       # Items processed per tick
decay.tickInterval = 256ms        # Base processing interval
decay.bucketInterval = 5min       # Timer wheel bucket size
decay.jitterMaxMs = 25            # ±25ms tick jitter

Projects/Server/Items/Item.cs

@@ -368,6 +368,7 @@ public partial class Item : IHued, IComparable<Item>, ISpawnable, IObjectPropert
                 }
 
                 Delta(ItemDelta.Update);
+                UpdateDecayRegistration();
             }
         }
     }
@@ -383,6 +384,7 @@ public partial class Item : IHued, IComparable<Item>, ISpawnable, IObjectPropert
                 SetFlag(ImplFlag.Movable, value);
 
                 Delta(ItemDelta.Update);
+                UpdateDecayRegistration();
             }
         }
     }
@@ -1436,6 +1438,7 @@ public partial class Item : IHued, IComparable<Item>, ISpawnable, IObjectPropert
         }
 
         OnDelete();
+        DecayScheduler.Unregister(this);
 
         var items = LookupItems();
 
@@ -1478,6 +1481,8 @@ public partial class Item : IHued, IComparable<Item>, ISpawnable, IObjectPropert
         ClearProperties();
     }
 
+    public virtual bool SkipSerialization => false;
+
     [IgnoreDupe]
     public ISpawner Spawner
     {
@@ -1485,6 +1490,7 @@ public partial class Item : IHued, IComparable<Item>, ISpawnable, IObjectPropert
         set
         {
             var info = AcquireCompactInfo();
+            var oldValue = info.m_Spawner;
 
             info.m_Spawner = value;
 
@@ -1492,6 +1498,11 @@ public partial class Item : IHued, IComparable<Item>, ISpawnable, IObjectPropert
             {
                 VerifyCompactInfo();
             }
+
+            if (oldValue != value)
+            {
+                UpdateDecayRegistration();
+            }
         }
     }
 
@@ -2268,9 +2279,22 @@ public partial class Item : IHued, IComparable<Item>, ISpawnable, IObjectPropert
     public virtual bool OnDecay() =>
         CanDecay() && Region.Find(Location, Map).OnDecay(this);
 
+    public DateTime ScheduledDecayTime => LastMoved + DecayTime;
+
+    public void UpdateDecayRegistration()
+    {
+        DecayScheduler.Unregister(this);
+
+        if (CanDecay())
+        {
+            DecayScheduler.Register(this);
+        }
+    }
+
     public void SetLastMoved()
     {
         LastMoved = Core.Now;
+        UpdateDecayRegistration();
     }
 
     public virtual bool CanStackWith(Item dropped) =>
@@ -3205,6 +3229,7 @@ public partial class Item : IHued, IComparable<Item>, ISpawnable, IObjectPropert
         }
 
         item.Delta(ItemDelta.Update);
+        item.UpdateDecayRegistration();
 
         item.OnAdded(this);
         OnItemAdded(item);
@@ -3374,6 +3399,7 @@ public partial class Item : IHued, IComparable<Item>, ISpawnable, IObjectPropert
             }
 
             item.Parent = null;
+            item.UpdateDecayRegistration();
 
             item.OnRemoved(this);
             OnItemRemoved(item);