[Phase 3] Write full API reference in README

[Robinsonchiziterem]

Summary

Document every public prop and type in the README with descriptions, type signatures, defaults, and examples — so a developer can integrate the component without reading source code.

Acceptance Criteria

• Props table covers all 7 RouteVisualizerProps fields with type, default, and description
• RouteData, RouteHop, and StellarAsset type shapes are shown as code blocks
• Quick-start example is copy-paste runnable
• Theming / CSS custom properties section is present

Drip Wave

Complexity: Medium — 150 pts
Phase: 3 — Production Hardening