fix: decouple MuJoCo import chain for optional Motrix paths

[TATP-233]

Summary

• lazy-load the MuJoCo backend from the public backend package instead of importing it at package import time
• remove top-level MuJoCo imports from motion-tracking and Allegro env import/bootstrap paths, and resolve MuJoCo-style body ids from XML for Motrix
• split pure config/bootstrap checks from MuJoCo runtime tests and document backend-specific runtime prerequisites

Fixes #151

Validation

• uv run pytest tests/envs/test_env_configs.py -q -m 'not slow and not veryslow'
• uv run pytest tests/envs/test_env_configs.py::test_registry_bootstrap_and_config_imports_do_not_require_mujoco -q
• uv run pytest 'tests/envs/test_env_configs.py::test_env_reset_and_step[Go1JoystickFlatTerrain]' -q -m slow
• uv run pytest 'tests/envs/test_env_configs.py::test_g1_motion_tracking_reset_and_step[mujoco]' -q -m slow
• uv run pytest tests/utils/test_algo_utils.py -q
• uv run pytest tests/scripts/test_check_docs.py -q
• uv run ruff check src/unilab/base/backend/init.py src/unilab/utils/xml_utils.py src/unilab/envs/motion_tracking/g1/tracking.py src/unilab/envs/manipulation/inhand_rot_allegro/base.py src/unilab/envs/manipulation/inhand_rot_allegro/gen_grasp.py tests/envs/test_env_configs.py
• uv run pyright src/unilab/base/backend/init.py tests/envs/test_env_configs.py
• uv run mypy src/unilab/base/backend/init.py