feat: sprite art pass (replace placeholder rectangles)

## Summary
Replace all placeholder rectangle graphics with actual pixel art sprites.

## Scope
- Player ship sprite (horizontal, R-Type inspired)
- Basic enemy sprite
- Boss sprites (3 phase variants)
- Bullet sprites (player vs enemy)
- Scrap pickup sprite
- Force pod sprite (for future mechanic)

## Notes
- Art style: MS Paint-adjacent, flat colors, chunky pixels (see DESIGN.md)
- Base resolution: 480x270 scaled up
- Depends on #1 (scaffold must be in place)