Implement Stellar Transfer Replay Protection Cache

# 🧠 Concept  
Cache replay protection metadata for Stellar transfers.

## ⚠️ Problem  
Replay validation may become inefficient at scale.

## 📁 Implementation Scope  
- `src/cache/replay/stellar/`

## ✅ Requirements  
- Store replay metadata  
- Support fast replay validation  

## 🎯 Acceptance Criteria  
- [ ] Replay cache implemented  