Implement Stellar Contract Risk Scoring Engine

# 🧠 Concept  
Generate security and optimization risk scores for Soroban contracts.

## ⚠️ Problem  
Developers need summarized audit insights.

## 📁 Implementation Scope  
- `src/scoring/stellar/`

## ✅ Requirements  
- Aggregate rule findings  
- Generate weighted risk scores  

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