[mujoco] add native frame_stack across Gym, DMC, and robotics

[Trinkle23897]

Summary

• Problem: MuJoCo frame stacking was missing from the native env observation path, so users needed Python-side wrappers and DMC/robotics did not preserve native multi-leaf observation structure.
• Scope: Adds native C++ frame_stack support for Gym MuJoCo, dm_control suite envs, and Gymnasium-Robotics envs. This does not add pixel observations to the main obs pipeline.
• Outcome: frame_stack > 1 prepends a stack dimension per observation leaf, reset fills the stack with the first frame, step rolls and appends the latest frame, and frame_stack = 1 preserves existing shapes and values.

This diff adds native frame stacking for MuJoCo state observations across Gym, DMC, and robotics env families.

Technical Details

• Approach: Add a shared MuJoCo frame-stack helper that keeps one rolling buffer per observation key, then wire each env family to call it from C++ WriteState.
• Code pointers:
  • envpool/mujoco/frame_stack.h: shared StackSpec and per-key FrameStackBuffer.
  • envpool/mujoco/gym/mujoco_env.h: Gym MuJoCo single-observation frame-stack path.
  • envpool/mujoco/dmc/mujoco_env.h: DMC multi-leaf observation frame-stack plumbing.
  • envpool/mujoco/robotics/mujoco_env.h: robotics multi-leaf observation frame-stack plumbing.
  • envpool/mujoco/robotics/kitchen.h: per-goal-leaf stack handling for Franka Kitchen.
• Notes: frame_stack = 1 is intentionally a no-op in both spec shape and write-path behavior.

Test Plan

Automated

• git diff --check: passed.
• bazel test //envpool/mujoco:mujoco_envpool_test: passed.
• bazel test //envpool/mujoco:mujoco_gym_deterministic_test --test_filter=frame_stack: passed.
• bazel test //envpool/mujoco:mujoco_dmc_suite_deterministic_test --test_filter=frame_stack: passed.
• bazel test //envpool/mujoco:robotics_test --test_filter=frame_stack: passed.

Suggested Manual

• python - <<'PY'
from envpool.registration import make_gymnasium
env = make_gymnasium("HalfCheetah-v5", num_envs=1, frame_stack=4)
obs, _ = env.reset()
print(obs.shape)
PY
  Verify a simple Gym MuJoCo env reports the expected stacked observation shape.