feat(client-java): add writeArrayElements default-fill helper and document semantics

clients/java/README.md

@@ -124,6 +124,29 @@ the unchanged elements included. For example, to change 2 elements of a
 the 2 new ones). Sending only the 2 changed values overwrites the attribute
 with a 2-element array.
 
+When only a few indices need changing and the rest should be reset to the
+element type's default, use `writeArrayElements` instead of building the full
+array manually:
+
+```java
+session.writeArrayElements(
+        serverHandle, itemHandle,
+        MxDataType.MX_DATA_TYPE_INTEGER,
+        20, // totalLength
+        Map.of(
+                2, MxValues.int32Value(42),
+                7, MxValues.int32Value(99)),
+        userId);
+```
+
+The gateway expands the sparse descriptor into a full `totalLength`-element
+array before forwarding to the worker. Indices not listed in the map are
+written as the element type's default — this is a **reset**, not a preserve;
+current values at those positions are discarded. `totalLength` is required and
+must match the declared length of the array attribute. Bare-name array items
+(`Area001.Pump001.Speed`) are auto-normalized to the `[]` form at `AddItem` so
+the array attribute accepts the write.
+
 ## Galaxy Repository Browse
 
 The Galaxy Repository service is a separate metadata-only gRPC service exposed