feat: Add death mechanics, signal system, and friendly fire prevention

 - Implement character death with visual         
 feedback                                        
 - Add line-of-sight signal system for squad     
 commands                                        
 - Track bullet shooters to prevent friendly     
 fire                                            
 - Add dead character assets                     
                                                 
 Changes include:                                
                                                 
 - Armed.py: Death state, visual feedback,       
   skip dead enemies                             
 - SquadMember.py: Signal/connection checking    
   through walls                                 
 - Bullet.py: Shooter tracking, team-based       
   targeting                                     
 - Wall.py: Signal blocking property             
 - Level.py: Connection checks for commands      
 - Utils.py & __init__.py: Dead character asset