Implement Stellar Route Risk Assessment Engine

# 🧠 Concept  
Assess security and reliability risks of Stellar bridge routes.

## ⚠️ Problem  
Unsafe bridge routes may expose users to failed or malicious transfers.

## 📁 Implementation Scope  
- `src/risk/routes/stellar/`

## ✅ Requirements  
- Analyze route reliability  
- Assign route risk scores  

## 🎯 Acceptance Criteria  
- [ ] Risk assessment engine implemented  