feat: add Export ReClass XML and remove local-path tests

Adds Export ReClass XML menu item that writes NodeTree to ReClass .NET
compatible XML format with full round-trip fidelity. Removes test cases
that referenced local machine file paths.

src/export_reclass_xml.h

@@ -0,0 +1,10 @@
+#pragma once
+#include "core.h"
+
+namespace rcx {
+
+// Export a NodeTree to ReClass .NET / ReClassEx compatible XML format.
+// Returns true on success; populates errorMsg on failure if non-null.
+bool exportReclassXml(const NodeTree& tree, const QString& filePath, QString* errorMsg = nullptr);
+
+} // namespace rcx