SpaceShooter placeholder sprite set

Files:
- player_ship.png
- alien_scout.png
- alien_saucer.png
- alien_heavy.png
- exhaust_1.png ... exhaust_4.png
- player_shot_1.png ... player_shot_3.png
- enemy_shot_1.png ... enemy_shot_3.png
- explosion_1.png ... explosion_4.png
- sprite_sheet_source.png

All individual sprites are RGBA PNG files on transparent, power-of-two canvases.
They are intended as placeholder art and can be replaced later without changing game logic.
