Implement Soroban Bridge Event Aggregator

# 🧠 Concept  
Aggregate Soroban bridge events into unified streams.

## ⚠️ Problem  
Bridge events are fragmented across integrations.

## 📁 Implementation Scope  
- `src/events/aggregation/stellar/`

## ✅ Requirements  
- Normalize event payloads  
- Stream aggregated events  

## 🎯 Acceptance Criteria  
- [ ] Event aggregator implemented  