fix: address SonarQube reliability issues

- ElkDiagramRenderer.getElkRoot(): add null guard to prevent NPE
  when node is null (SQ java:S2259)
- WriteBuffer: add offerOrWarn() that logs when buffer is full instead
  of silently dropping data. ChunkAccumulator now uses this method
  so ingestion backpressure is visible in logs (SQ java:S899)

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

cameleer3-server-app/src/main/java/com/cameleer3/server/app/diagram/ElkDiagramRenderer.java

@@ -884,6 +884,7 @@ public class ElkDiagramRenderer implements DiagramRenderer {
     }
 
     private ElkNode getElkRoot(ElkNode node) {
+        if (node == null) return null;
         ElkNode current = node;
         while (current.getParent() != null) {
             current = current.getParent();