Build Score Tracker Module

[Mkalbani]

Description

Implement a score module that calculates and persists player scores based on puzzle performance.

Acceptance Criteria

• A score module exists under src/score/mod.rs
• Score increases based on puzzle difficulty and time taken
• High score is tracked and updated per session
• Score can be serialized for persistence
• Unit tests validate scoring formulas and high score updates

[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/