Fix InstancedNode losing InstanceNodeControl after deserialization

[Copilot]

InstancedNode loses its InstanceNodeControl after serialization/deserialization, leaving the node non-functional. The control's node field was not being serialized, resulting in a null reference on load.

InstancedNode node = new InstancedNode("test");
// ... populate with instanced geometries ...
node.instance();

InstancedNode loaded = (InstancedNode) BinaryExporter.saveAndLoad(assetManager, node);
loaded.getNumControls(); // returns 0 - control is missing

Changes

• InstancedNode.java: Override write(JmeExporter) and read(JmeImporter) to explicitly serialize/deserialize the control field
• Pattern mirrors existing CameraNode and LightNode implementations
• InstancedNodeTest.java: Verify control preservation across serialization

The inner InstancedNodeControl class has empty write/read methods, so while the control was being serialized through the parent Spatial class's controls list, its internal node reference was not. Explicit serialization at the InstancedNode level resolves this.

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Original prompt

---

This section details on the original issue you should resolve

<issue_title>InstancedNode: missing InstanceNodeControl after deserialization</issue_title>
<issue_description>When deserializing a InstancedNode, the associated InstanceNodeControl is missing.

https://github.com/jMonkeyEngine/jmonkeyengine/blob/master/jme3-core/src/main/java/com/jme3/scene/instancing/InstancedNode.java#L124

Minimal Example:
The following code creates an InstancedNode, populates it with many instanced geometries, and then serializes/deserializes it using BinaryExporter.saveAndLoad.
After loading, the InstancedNode no longer contains its InstanceNodeControl.

        InstancedNode instancedNode = new InstancedNode("instanced_node");

        int extent = 30;
        Material material = new Material(assetManager, "Common/MatDefs/Misc/Unshaded.j3md");
        material.setBoolean("UseInstancing", true);
        material.setColor("Color", ColorRGBA.Red);

        for (int y = -extent; y < extent; y++) {
            for (int x = -extent; x < extent; x++) {
                Geometry geo = createInstance(x, y);
                geo.setMaterial(material);
                instancedNode.attachChild(instance);
            }
        }

        instancedNode.instance();
        rootNode.attachChild(instancedNode);

        InstancedNode load = (InstancedNode) BinaryExporter.saveAndLoad(assetManager, instancedNode);
        load.getNumControls(); // returns 0
```</issue_description>

<agent_instructions>investigate the issue and figure out if it needs a patch and how to patch it properly</agent_instructions>

## Comments on the Issue (you are @copilot in this section)

<comments>
</comments>

• Fixes InstancedNode: missing InstanceNodeControl after deserialization #2592

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