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
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