Add headless EGL pixel rendering benchmark

[eust-w]

Summary

• document accelerated headless pixel rendering with MUJOCO_GL=egl
• document MUJOCO_EGL_DEVICE_ID selection for multi-GPU hosts
• clarify that BiGym's built-in MuJoCo renderer uses OpenGL/EGL, not CUDA or ROCm
• add a deterministic pixel-observation smoke benchmark with JSON throughput output
• fail the benchmark on wrong-shaped, non-finite, blank or constant frames
• add focused validation tests for the render-output gate

Why

BiGym documents pixel observations but does not currently provide a reproducible way to validate headless GPU rendering. This makes blank frames, software-rendering fallbacks and container device-permission problems difficult to distinguish from environment or policy failures.

The new example exercises a real ReachTarget environment with rgb_head observations, reports physics-plus-rendering steps per second, and produces a small machine-readable result that can be compared across hosts.

AMD and portability boundary

On AMD Linux hosts, MuJoCo normally reaches the GPU through Mesa EGL/OpenGL and /dev/dri; installing ROCm is not required for BiGym's built-in pixel observations. External policies and renderers may still require ROCm, but those dependencies remain outside BiGym.

The same example works for other Linux EGL devices. MUJOCO_GL=osmesa remains documented as a CPU fallback, not a GPU performance result. MUJOCO_EGL_DEVICE_ID follows the device-selection support in the official MuJoCo Python EGL implementation.

Downstream Radeon W7900D integration evidence exists at https://github.com/eust-w/amd-bigym-openpi-eval/blob/main/evidence/amd-full-recorder-smoke-20260805.json, but this PR does not present that forked/custom-renderer run as an execution of this exact canonical example.

Validation

• 4 focused tests passed for valid, blank, mis-shaped and non-finite frames
• Black 23.3.0, Ruff 0.0.272 and pydocstyle 6.3.0 passed
• Python compilation and git diff checks passed
• no new runtime dependency is introduced

Supersedes #58 after the source branch was renamed to remove the agent/ prefix.