Add MuJoCo Playground environments

[Trinkle23897]

Summary

• add native EnvPool support for MuJoCo Playground v0.2.0 non-DM-Control tasks: 19 locomotion + 10 manipulation envs
• vendor pinned Playground/Menagerie assets and patches under third_party/mujoco_playground
• add registry, deterministic, pixel-only, oracle alignment, and render comparison coverage
• document supported task IDs and add EnvPool-vs-official render comparison image

Notes

• MuJoCo Playground vendored DM Control Suite tasks are intentionally not registered here; EnvPool already exposes them through the existing dm_control family.

Local validation

• python3 -m ruff check scripts/render_compare.py envpool/mujoco/playground/playground_oracle_align_test.py envpool/mujoco/playground/playground_test.py envpool/mujoco/playground/registration.py envpool/mujoco/playground/init.py
• PYTHONPYCACHEPREFIX=/private/tmp/envpool-pycache python3 -m py_compile scripts/render_compare.py envpool/mujoco/playground/playground_oracle_align_test.py envpool/mujoco/playground/playground_test.py envpool/mujoco/playground/registration.py envpool/mujoco/playground/init.py
• git diff --check
• bazel test --config=debug //envpool/mujoco:playground_test //envpool/mujoco:playground_pixel_observation_test //envpool/mujoco:playground_oracle_align_test --test_output=errors

[chatgpt-codex-connector]

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[chatgpt-codex-connector]

Set EGL_PLATFORM when selecting EGL backend

On headless Linux systems that require surfaceless EGL, this helper can mis-detect a usable EGL setup because it probes with MUJOCO_GL=egl but never sets EGL_PLATFORM=surfaceless, and it also exits early when MUJOCO_GL is already set (without normalizing EGL_PLATFORM). In that case the probe may fail, no backend is selected, and later MuJoCo rendering can fail even though EGL would work with the correct platform setting.

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