Add system mdspan CMake option (#1403)

Summary:
- Add a `MOMENTUM_USE_SYSTEM_MDSPAN` CMake option, defaulting to `OFF`.
- Use `find_package(mdspan CONFIG REQUIRED)` when that option is enabled.
- Preserve the existing `FetchContent` mdspan path by default for source builds.

## Motivation
Package-manager builds should be able to use the mdspan package supplied by the environment instead of downloading it with CMake `FetchContent`. This keeps conda-forge and similar builds offline-friendly, reproducible, and aligned with their declared dependency graph, while leaving the default source-build path convenient for users who do not already have mdspan installed.

This is needed by the conda-forge packaging flow today: `conda-forge/momentum-feedstock` currently carries a downstream patch to replace the mdspan `FetchContent` block with `find_package(mdspan CONFIG REQUIRED)`:

- conda-forge PR: conda-forge/momentum-feedstock#292
- downstream patch permalink: https://github.com/conda-forge/momentum-feedstock/blob/8abdac65ab4781261eb856bdcb3b2e66e2b09964/recipe/patches/0003-Use-packaged-mdspan.patch
- recipe dependency permalink: https://github.com/conda-forge/momentum-feedstock/blob/8abdac65ab4781261eb856bdcb3b2e66e2b09964/recipe/recipe.yaml
- conda-forge mdspan feedstock: https://github.com/conda-forge/mdspan-feedstock

With this option upstream, conda-forge can eventually drop that downstream patch after a Momentum release and pass `-DMOMENTUM_USE_SYSTEM_MDSPAN=ON` instead.

Pull Request resolved: #1403

Test Plan:
- `git diff --check origin/main..HEAD`
- `pixi run -e py312 cmake -S . -B build/pr-mdspan-off-clean ... -DMOMENTUM_USE_SYSTEM_MDSPAN=OFF`
- `CMAKE_PREFIX_PATH=<conda-forge mdspan prefix> pixi run -e py312 cmake -S . -B build/pr-mdspan-on-clean ... -DMOMENTUM_USE_SYSTEM_MDSPAN=ON`
- `pixi run -e py312 cmake --build build/pr-mdspan-on-clean --target rasterizer -j4`

Reviewed By: jeongseok-meta

Differential Revision: D105652029

fbshipit-source-id: 23d58194b903b164d841e788c25e0a698f5b2739

Author: jeongseok-meta

CMakeLists.txt

@@ -88,6 +88,7 @@ mt_option(MOMENTUM_ENABLE_PROFILING "Enable building with profiling annotations"
 mt_option(MOMENTUM_ENABLE_SIMD "Enable building with SIMD instructions" ON)
 mt_option(MOMENTUM_INSTALL_EXAMPLES "Install examples" OFF)
 mt_option(MOMENTUM_USE_SYSTEM_GOOGLETEST "Use GoogleTest installed in system" OFF)
+mt_option(MOMENTUM_USE_SYSTEM_MDSPAN "Use mdspan installed in system" OFF)
 mt_option(MOMENTUM_USE_SYSTEM_PYBIND11 "Use pybind11 installed in system" OFF)
 mt_option(MOMENTUM_USE_SYSTEM_RERUN_CPP_SDK "Use Rerun C++ SDK installed in system" OFF)
 mt_option(MOMENTUM_USE_SYSTEM_TRACY "Use Tracy installed in system" OFF)
@@ -132,17 +133,21 @@ find_package(re2 MODULE REQUIRED)
 find_package(spdlog CONFIG REQUIRED)
 find_package(urdfdom CONFIG REQUIRED)
 
-# Use standalone mdspan (header-only) instead of full Kokkos.
-# Full Kokkos with CUDA sets CMAKE_CXX_COMPILER to nvcc_wrapper globally,
-# which breaks pybind11 compilation. We only need mdspan headers.
-include(FetchContent)
-FetchContent_Declare(mdspan
-  URL https://github.com/kokkos/mdspan/archive/refs/heads/stable.tar.gz
-)
-set(MDSPAN_ENABLE_TESTS OFF CACHE BOOL "" FORCE)
-set(MDSPAN_ENABLE_BENCHMARKS OFF CACHE BOOL "" FORCE)
-set(MDSPAN_ENABLE_EXAMPLES OFF CACHE BOOL "" FORCE)
-FetchContent_MakeAvailable(mdspan)
+if(MOMENTUM_USE_SYSTEM_MDSPAN)
+  find_package(mdspan CONFIG REQUIRED)
+else()
+  # Use standalone mdspan (header-only) instead of full Kokkos.
+  # Full Kokkos with CUDA sets CMAKE_CXX_COMPILER to nvcc_wrapper globally,
+  # which breaks pybind11 compilation. We only need mdspan headers.
+  include(FetchContent)
+  FetchContent_Declare(mdspan
+    URL https://github.com/kokkos/mdspan/archive/refs/heads/stable.tar.gz
+  )
+  set(MDSPAN_ENABLE_TESTS OFF CACHE BOOL "" FORCE)
+  set(MDSPAN_ENABLE_BENCHMARKS OFF CACHE BOOL "" FORCE)
+  set(MDSPAN_ENABLE_EXAMPLES OFF CACHE BOOL "" FORCE)
+  FetchContent_MakeAvailable(mdspan)
+endif()
 
 if(MOMENTUM_BUILD_RERUN)
   if(MOMENTUM_USE_SYSTEM_RERUN_CPP_SDK)