s&box UndoSystem: per-player stacks keyed by SteamId, bounded at 128, RemovePlayer on disconnect, Rpc.FilterInclude for notices

Sandbox-style games need per-player undo stacks. Common mistakes: (1) not calling RemovePlayer on disconnect causing memory leaks, (2) unbounded stack growth, (3) broadcasting undo notices to all players instead of just the owner, (4) not removing objects from undo stacks when they're picked up (allowing players to undo weapons out of other players' hands).