Package Filament runtime assets

[devshahofficial]

Split out from #3254 in response to Haroon's request for smaller Filament PRs.

Summary

• Copies generated Filament .filamat and .ktx assets into Python wheel package data when present.
• Installs generated Filament runtime assets under ${CMAKE_INSTALL_LIBDIR}/assets.
• Narrows Linux plugin library matching to shared objects.
• Makes the classic Python renderer tolerate builds where GLContext is intentionally unavailable.
• Documents the Linux mixed-stdlib source-build caveat for Studio.

Validation

• git diff --check
• python3 -m py_compile python/setup.py python/mujoco/rendering/classic/renderer.py python/mujoco/renderer_test.py
• cmake -S . -B /tmp/mujoco-packaging-runtime-build -DMUJOCO_BUILD_TESTS=OFF -DMUJOCO_TEST_PYTHON_UTIL=OFF -DCMAKE_BUILD_TYPE=Release
• cmake --build /tmp/mujoco-packaging-runtime-build --target mujoco -j2

Notes

• Draft PR: wheel-building with generated Filament assets should still be run in CI / a Filament-enabled environment.