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

cmake/third_party_deps/openusd/CMakeLists.txt

@@ -13,42 +13,39 @@
 # limitations under the License.
 cmake_minimum_required(VERSION "3.16")
 
-option(BUILD_USD OFF "Build OpenUSD")
 project(openusd-dependency)
 
-if (BUILD_USD)
-  include(ExternalProject)
+include(ExternalProject)
 
-  set(DEPS_DIR ${CMAKE_BINARY_DIR}/../../../../build/_deps)
+set(DEPS_DIR ${CMAKE_BINARY_DIR}/../../../../build/_deps)
 
-  ExternalProject_Add(openusd
-    GIT_REPOSITORY https://github.com/PixarAnimationStudios/USD.git
-    GIT_TAG v25.11
-    PREFIX openusd
-    BUILD_IN_SOURCE 0
-    UPDATE_COMMAND ""
-    CONFIGURE_COMMAND ""
-    INSTALL_COMMAND ""
-    SOURCE_DIR ${DEPS_DIR}/openusd-src
-    BINARY_DIR ${DEPS_DIR}/openusd-build
-    INSTALL_DIR ${DEPS_DIR}/openusd-build
-    BUILD_COMMAND python3 ${DEPS_DIR}/openusd-src/build_scripts/build_usd.py
-        --build-shared
-        --no-examples
-        --no-tools
-        --no-ptex
-        --no-prman
-        --no-openimageio
-        --no-opencolorio
-        --no-alembic
-        --no-draco
-        --no-materialx
-        --no-tutorials
-        --no-tests
-        --no-docs
-        --no-imaging
-        --no-python
-        --no-usdValidation
-        <INSTALL_DIR>
-  )
-endif()
+ExternalProject_Add(openusd
+  GIT_REPOSITORY https://github.com/PixarAnimationStudios/USD.git
+  GIT_TAG v25.11
+  PREFIX openusd
+  BUILD_IN_SOURCE 0
+  UPDATE_COMMAND ""
+  CONFIGURE_COMMAND ""
+  INSTALL_COMMAND ""
+  SOURCE_DIR ${DEPS_DIR}/openusd-src
+  BINARY_DIR ${DEPS_DIR}/openusd-build
+  INSTALL_DIR ${DEPS_DIR}/openusd-build
+  BUILD_COMMAND python3 ${DEPS_DIR}/openusd-src/build_scripts/build_usd.py
+      --build-shared
+      --no-examples
+      --no-tools
+      --no-ptex
+      --no-prman
+      --no-openimageio
+      --no-opencolorio
+      --no-alembic
+      --no-draco
+      --no-materialx
+      --no-tutorials
+      --no-tests
+      --no-docs
+      --no-imaging
+      --no-python
+      --no-usdValidation
+      <INSTALL_DIR>
+)