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>

cameleer3-server-app/src/main/java/com/cameleer3/server/app/config/DiagramBeanConfig.java

@@ -0,0 +1,18 @@
+package com.cameleer3.server.app.config;
+
+import com.cameleer3.server.app.diagram.ElkDiagramRenderer;
+import com.cameleer3.server.core.diagram.DiagramRenderer;
+import org.springframework.context.annotation.Bean;
+import org.springframework.context.annotation.Configuration;
+
+/**
+ * Creates beans for diagram rendering.
+ */
+@Configuration
+public class DiagramBeanConfig {
+
+    @Bean
+    public DiagramRenderer diagramRenderer() {
+        return new ElkDiagramRenderer();
+    }
+}