Implement production-ready game level system

Request to design and implement a robust, scalable game level system for Tower Game and prepare it for production deployment.

**Requirements:**
- Architect a level system suitable for both current gameplay and future expansions.
- Ensure the system supports saving/loading player progress reliably.
- Integrate level progression, rewards, and difficulty scaling.
- Conduct thorough testing and code review to guarantee production stability.
- Document the design, usage, and integration steps for future contributors.

**Goal:**
Make the level system production ready so it can support live players and future updates with minimal friction.