Break out struct initialization functions into engine_init.c.

PiperOrigin-RevId: 811598048
Change-Id: Ia832a8268e2209a5a264bbf79aa00dda487206bc

src/engine/engine_init.h

@@ -0,0 +1,35 @@
+// Copyright 2025 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_ENGINE_ENGINE_INIT_H_
+#define MUJOCO_SRC_ENGINE_ENGINE_INIT_H_
+
+#include <mujoco/mujoco.h>
+
+// set default options for length range computation
+void mj_defaultLROpt(mjLROpt* opt);
+
+// set model options to default values
+void mj_defaultOption(mjOption* opt);
+
+// set visual options to default values
+void mj_defaultVisual(mjVisual* vis);
+
+// set statistics to default values
+void mj_defaultStatistic(mjStatistic* stat);
+
+// set solver parameters to default values
+void mj_defaultSolRefImp(mjtNum* solref, mjtNum* solimp);
+
+#endif  // MUJOCO_SRC_ENGINE_ENGINE_INIT_H_