s&box Undo System: Per-Player Hierarchical Action Tracking

Implementing undo functionality in s&box requires tracking spawned objects and allowing players to revert their actions. Common challenges include:

1. Tracking object hierarchy for complex spawns (duplicator)
2. Handling object destruction mid-undo
3. Persisting undo across player death
4. Managing undo stack size and memory
5. Network synchronization for multiplayer