feat(02-02): implement ELK diagram renderer with SVG/JSON content negotiation
- ElkDiagramRenderer: ELK layered layout (top-to-bottom) with JFreeSVG rendering
- Color-coded nodes: blue endpoints, green processors, red errors, purple EIPs, cyan cross-route
- Compound node support for CHOICE/SPLIT/TRY_CATCH swimlane groups
- DiagramRenderController: GET /api/v1/diagrams/{hash}/render with Accept header negotiation
- DiagramBeanConfig for Spring wiring
- 11 unit tests (layout, SVG structure, colors, compound nodes)
- 4 integration tests (SVG, JSON, 404, default format)
- Added xtext xbase lib dependency for ELK compatibility
Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>
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