Move OpenUSD parsing to usd_decoder

Add usd_decoder to mujoco/plugin.
  - Previously was src/experimental/usd/usd_to_mjspec.cc, now that same spec is returned by the plugin.

PiperOrigin-RevId: 854350962
Change-Id: Ia980190a557c50ce8197980922a2594b613859b4

plugin/usd_decoder/CMakeLists.txt

@@ -0,0 +1,53 @@
+# 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
+#
+#   https://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.
+find_package(pxr REQUIRED)
+
+set(CMAKE_INSTALL_RPATH_USE_LINK_PATH TRUE)
+set(USD_DECODER_SRCS
+    usd_decoder.cc
+    kinematic_tree.cc
+    kinematic_tree.h
+    material_parsing.cc
+    material_parsing.h
+    utils.h
+)
+
+add_library(usd_decoder_plugin SHARED ${USD_DECODER_SRCS})
+target_include_directories(usd_decoder_plugin PRIVATE
+  ${CMAKE_CURRENT_SOURCE_DIR}/../..
+)
+target_link_libraries(usd_decoder_plugin PRIVATE
+  mujoco
+  mjcPhysics
+)
+target_compile_options(usd_decoder_plugin PRIVATE
+  ${AVX_COMPILE_OPTIONS}
+  ${MUJOCO_MACOS_COMPILE_OPTIONS}
+  ${EXTRA_COMPILE_OPTIONS}
+  ${MUJOCO_CXX_FLAGS}
+  -Wno-deprecated # pxr Tf lib uses deprecated header
+)
+target_link_options(usd_decoder_plugin PRIVATE
+  ${MUJOCO_MACOS_LINK_OPTIONS}
+  ${EXTRA_LINK_OPTIONS}
+)
+
+# Install to mujoco_plugin directory in bin location so that it is picked up by simulate
+# on startup.
+install(
+  TARGETS usd_decoder_plugin
+  LIBRARY DESTINATION "${CMAKE_INSTALL_BINDIR}/mujoco_plugin"
+)
+
+target_link_libraries(usd_decoder_plugin PRIVATE usd usdGeom usdPhysics usdShade gf tf ar vt kind)