Build Leaderboard Module

[Mkalbani]

Description

Implement a leaderboard module that ranks players by score locally and prepares data for on-chain submission.

Acceptance Criteria

• A leaderboard module exists under src/leaderboard/mod.rs
• Leaderboard stores top N scores with player id and timestamp
• New entries replace lower scores when the board is full
• Board is serializable for persistence or on-chain export
• Unit tests cover insertion, replacement, and ranking order

[ayomideadeniran]

I'm a full-stack developer and I'd like to take this issue.
I understand the requirements and will follow the existing project structure, keep the implementation clean, and include proper tests where applicable.

ETA: 1–3 days (depending on scope)
Portfolio: https://knightsdev-portfolio.vercel.app/