Implement Soroban Contract Call Graph Generator

# 🧠 Concept  
Generate function call graphs for Soroban contracts.

## ⚠️ Problem  
Complex call flows are difficult to understand manually.

## 📁 Implementation Scope  
- `src/graphs/stellar/call-graph/`

## ✅ Requirements  
- Detect function relationships  
- Export graph structure  

## 🎯 Acceptance Criteria  
- [ ] Call graph generated  