Implement Save/Load State Module

[Mkalbani]

Description

Build a persistence module that saves and restores full game state to disk.

Acceptance Criteria

• A persistence module exists under src/persistence/mod.rs
• Game state serializes to JSON via serde
• Save writes to a configurable file path
• Load deserializes and validates state before returning
• Unit tests cover save, load, corrupted file, and missing file scenarios

[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/