Implement Soroban AST Visualization Exporter

[mijinummi]

🧠 Concept

Export visual representations of parsed AST structures.

⚠️ Problem

AST structures are difficult to inspect manually.

📁 Implementation Scope

• src/visualization/stellar/ast/

✅ Requirements

• Generate graph exports
• Support multiple output formats
• Handle large AST structures

🎯 Acceptance Criteria

• AST visualization implemented
• Exports generated successfully