Implement Soroban Analysis Artifact Storage

# 🧠 Concept  
Persist generated analysis artifacts.

## ⚠️ Problem  
Reports and findings are lost after scan completion.

## 📁 Implementation Scope  
- `src/storage/artifacts/stellar/`

## ✅ Requirements  
- Store reports and metadata  
- Support artifact retrieval  

## 🎯 Acceptance Criteria  
- [ ] Artifact storage implemented  