docs(asteroids): document sound effects and Erik's swap! code review

Document the Web Audio API sound system implementation and Erik's
excellent feedback on atomic state updates.

Changes:
- Add comprehensive 'Retro Sound Effects' section with examples
- Document all 6 sound effects (laser, explosion, thrust, hyperspace, hit, victory)
- Show sound integration patterns and throttling techniques
- Add Erik's code review feedback to Hyperspace section
- Demonstrate improved single swap! pattern vs multiple swaps
- Update metadata with audio tags and keywords
- Enhance Learning Points, Key Takeaways, and Technical Highlights
- Update 'Extending the Game' section (audio now implemented)

Credits Erik for identifying the multiple swap! issue and explaining
the benefits of atomic state updates.

Author: burinc