Add packaged asset resource provider

[devshahofficial]

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

Summary

• Adds a default mujoco: resource provider for packaged runtime assets.
• Searches explicit asset env vars first, then assets next to the MuJoCo library, then local assets/.
• Rejects unsafe absolute and parent-directory asset names.
• Adds focused resource-provider tests.

Validation

• git diff --check
• cmake -S . -B /tmp/mujoco-resource-provider-build -DMUJOCO_BUILD_TESTS=ON -DMUJOCO_TEST_PYTHON_UTIL=OFF -DCMAKE_BUILD_TYPE=Release
• cmake --build /tmp/mujoco-resource-provider-build --target user_resource_test -j2
• /tmp/mujoco-resource-provider-build/bin/user_resource_test from the repository test/ cwd

Notes

• Draft PR: this is intended as the packaged-asset lookup slice of Prototype Filament package assets and renderer API #3254.

[havess]

Hey @devshahofficial ! Thanks for the PR. Would it be possible to move the plugin into our (plugins directory)[https://github.com/google-deepmind/mujoco/tree/main/plugin] instead?

I try to steer clear of special branches for specific resource loading/file support in user/ when I can.

[devshahofficial]

Thanks @havess, done. I moved the mujoco: asset provider into plugin/resource_provider and removed the special-case registration path from src/user/user_resource.cc.

Validated with:

• git diff --check
• cmake -S . -B /tmp/mujoco-pr3339-build -DMUJOCO_BUILD_TESTS=ON -DMUJOCO_TEST_PYTHON_UTIL=OFF -DCMAKE_BUILD_TYPE=Release
• cmake --build /tmp/mujoco-pr3339-build --target user_resource_test -j2
• /tmp/mujoco-pr3339-build/bin/user_resource_test from test/ (20 passed)

[devshahofficial]

Quick CI note: the lone macos-15-x86_64 failure looks infra-related rather than code-related. Configure failed while FetchContent was cloning GLFW because the runner could not resolve github.com:

fatal: unable to access 'https://github.com/glfw/glfw.git/': Could not resolve host: github.com

The follow-up artifact upload also failed with ENOTFOUND. I tried rerunning the failed run/job from gh, but GitHub rejected the rerun request, so this likely needs a maintainer-side rerun if the check does not get retried automatically.

[devshahofficial]

Pushed an empty Trigger CI rerun commit because GitHub would not let me rerun the infra-failed macOS job directly. No source changes in this push; just trying to get a clean fresh matrix after the DNS failure.

[devshahofficial]

@havess @mmossg I moved the mujoco: provider into plugin/resource_provider from the earlier note and reran CI; the matrix is green now.

I would land this after #3337. Could you take another look when you have bandwidth?