Improve Filament Studio source builds

[devshahofficial]

Summary

This draft improves the experimental Studio + Filament source-build path in two ways:

• prevents Filament's Abseil lookup from discovering unrelated package-manager CMake configs after MuJoCo has already fetched Abseil
• documents a minimal isolated Studio build/run flow, including macOS runtime expectations and troubleshooting for conflicting CMake package prefixes

Motivation

While validating mujoco_studio with MUJOCO_USE_FILAMENT=ON, configure can fail on machines with ambient package-manager CMake configs, for example an Anaconda-provided abslConfig.cmake. Filament's FILAMENT_USE_EXTERNAL_ABSL path calls find_package(absl), which can collide with the Abseil targets already created by MuJoCo's fetched dependency.

This keeps the Filament fetch isolated from unrelated Abseil package configs, while preserving any pre-existing CMAKE_DISABLE_FIND_PACKAGE_absl value after Filament is configured.

Validation

• git diff --check
• fresh CMake configure of Studio + Filament without manually passing -DCMAKE_DISABLE_FIND_PACKAGE_absl=TRUE
• cmake --build build-studio-filament-eval --target mujoco_studio --parallel $(sysctl -n hw.logicalcpu)
• local runtime smoke test:
  • ./mujoco_studio --gfx=opengl --model_file=/tmp/mujoco_go2_demo/unitree_go2/scene_filament_open_assets_showcase.xml
  • runtime log: FEngine resolved backend: OpenGL
  • platform log: Apple M3 Pro, OpenGL reported through Apple's Metal-backed OpenGL layer

Screenshot from the local smoke test:

Related context

Tagging a few people from the surrounding Filament/Studio discussion for visibility:

• @yuvaltassa noted in issue Renderer API #2487 that Studio is close to ready from src/experimental/studio/: Renderer API #2487 (comment). This PR is a small source-build/doc cleanup around that path.
• @BlGene and @wpumacay have related downstream wheel/runtime requirements from the AllenAI/MolmoSpaces work: Renderer API #2487 (comment), Renderer API #2487 (comment), and https://github.com/allenai/mujoco/tree/molmospaces.
• @julien-blanchon has related prior work around Filament builds, wheel asset staging, and runtime asset lookup: Renderer API #2487 (comment) and https://github.com/julien-blanchon/mujoco/tree/exp-filament-build.

Notes

The Go2 / Filament sample-asset scene used for the runtime smoke test is local-only and not included in this PR.

[devshahofficial]

Keeping this PR focused on the Studio source-build path. I opened a separate stacked draft for the packaging/runtime follow-up pieces: generated Filament assets in install/wheels, default runtime asset lookup, and a small renderer identity query surfaced through Python.

Follow-up draft: #3254

That follow-up is still draft because full wheel artifact verification is pending, but I smoke-tested Studio with a Go2 scene on macOS and verified the MJR-compat build exports mjr_getRenderer() returning filament.