s&box Spawner System: ISpawner Interface with Preview and Undo

Implementing a flexible spawning system in s&box requires handling different spawn types (props, entities, dupes) with preview visualization, placement validation, and undo integration. Common challenges include:

1. Creating a unified spawner interface for different object types
2. Implementing real-time preview with proper materials
3. Handling rotation and snap placement
4. Integrating with the undo system
5. Network-syncing spawner state across clients