feat: add `get_top_referrers` leaderboard

[sanmipaul]

get_stats returns data for a single referrer. A referral program driving growth needs an aggregate leaderboard view, which currently requires indexing every register/record_activity event off-chain.

[devemmanuel28]

Hi maintainer,

I'd like to work on this issue.

I've reviewed the requirements and understand the goal is to add an efficient on-chain get_top_referrers leaderboard so clients can retrieve the highest-performing referrers without scanning every referral event off-chain.

My approach will be to design a storage structure that maintains referral rankings as activity is recorded, keeping leaderboard queries efficient while preserving the existing get_stats functionality. I'll ensure the implementation scales well, integrates cleanly with the current contract architecture, and adds comprehensive tests covering ranking updates, ordering, tie cases, and edge conditions.

I have experience building Soroban smart contracts in Rust and implementing protocol features with a strong focus on correctness, performance, and maintainability. I'll follow the project's conventions and deliver a clean, well-tested implementation.

[grantfox-oss]

🦊 GrantFox — @devemmanuel28 has been assigned to this issue as part of the Official Campaign | FWC26 campaign!

Next steps:

1. Open a Pull Request referencing this issue (e.g., Closes #943)
2. Your PR will be reviewed by the astera-hq maintainers

Good luck! Track your progress on GrantFox.

[grantfox-oss]

🎉 This issue has been marked as completed on GrantFox as part of the Official Campaign | FWC26 campaign!

@devemmanuel28's PR #1031 was approved and merged by @sanmipaul.

🏆 @devemmanuel28: You earned 35 FoxPoints for this contribution! Your current tier: Explorer (80 total points). Track your full progress on GrantFox.

👏 Great work, @devemmanuel28! Keep contributing to astera-hq.