Implement Soroban Route Cost Optimization Engine

[mijinummi]

🧠 Concept

Optimize route selection for lowest total transfer cost.

⚠️ Problem

Users often prioritize minimizing bridge and network fees.

📁 Implementation Scope

• src/optimization/costs/stellar/

✅ Requirements

• Evaluate route costs
• Rank low-cost routes
• Support configurable cost weights

🎯 Acceptance Criteria

• Cost optimization engine implemented
• Routes ranked by total cost