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/Serialization/GenericEntityPersistence.cs

@@ -98,10 +98,19 @@ public class GenericEntityPersistence<T> : GenericPersistence, IGenericEntityPer
         }
 
         idx.Write(3); // Version
-        idx.Write(EntitiesBySerial.Values.Count);
 
+        var countPosition = idx.Position;
+        idx.Write(0);
+
+        var entityCount = EntitiesBySerial.Count;
         foreach (var e in EntitiesBySerial.Values)
         {
+            if (e is Item { SkipSerialization: true } or Mobile { SkipSerialization: true })
+            {
+                entityCount--;
+                continue;
+            }
+
             var thread = e.SerializedThread;
             var heapStart = e.SerializedPosition;
             var heapLength = e.SerializedLength;
@@ -130,6 +139,11 @@ public class GenericEntityPersistence<T> : GenericPersistence, IGenericEntityPer
 
             binPosition += heapLength;
         }
+
+        var currentPosition = idx.Position;
+        idx.Seek(countPosition, SeekOrigin.Begin);
+        idx.Write(entityCount);
+        idx.Seek(currentPosition, SeekOrigin.Begin);
     }
 
     public override void Serialize()