Move GlobalModel class and functionality into its own file.

We'll need some of this functionality for tests.

PiperOrigin-RevId: 853652149
Change-Id: Ib924d886b73784a0f370285e0b82b2ed833e3e77

src/xml/xml_global.h

@@ -0,0 +1,27 @@
+// Copyright 2026 DeepMind Technologies Limited
+//
+// Licensed under the Apache License, Version 2.0 (the "License");
+// you may not use this file except in compliance with the License.
+// You may obtain a copy of the License at
+//
+//     http://www.apache.org/licenses/LICENSE-2.0
+//
+// Unless required by applicable law or agreed to in writing, software
+// distributed under the License is distributed on an "AS IS" BASIS,
+// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+// See the License for the specific language governing permissions and
+// limitations under the License.
+
+
+#ifndef MUJOCO_SRC_XML_XML_GLOBAL_H_
+#define MUJOCO_SRC_XML_XML_GLOBAL_H_
+
+#include <string>
+
+#include <mujoco/mujoco.h>
+
+void SetGlobalXmlSpec(mjSpec* spec = nullptr);
+
+std::string GetGlobalXmlSpec(const mjModel* m, char* error, int error_sz);
+
+#endif  // MUJOCO_SRC_XML_XML_GLOBAL_H_