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-core/src/main/java/com/cameleer3/server/core/ingestion/WriteBuffer.java

@@ -38,6 +38,16 @@ public class WriteBuffer<T> {
         return queue.offer(item);
     }
 
+    /**
+     * Offer an item, logging a warning if the buffer is full.
+     * Use this as a {@code Consumer<T>} when the caller cannot handle backpressure.
+     */
+    public void offerOrWarn(T item) {
+        if (!queue.offer(item)) {
+            log.warn("WriteBuffer full (capacity={}), item dropped", capacity);
+        }
+    }
+
     /**
      * Offer a batch of items with all-or-nothing semantics.
      * If the buffer does not have enough remaining capacity for the entire batch,