fix: materialize MuJoCo pool outside hot path

[TATP-233]

Summary

• materialize MuJoCo BatchEnvPool explicitly after init domain randomization instead of lazy hot-path creation
• keep init randomization before pool materialization so model variants do not require duplicate pool builds
• make Allegro missing grasp cache fall back to procedural reset and update slow tests for the lifecycle contract

Validation

• UV_CACHE_DIR=/tmp/uv-cache uv run pytest -m "slow and not veryslow" tests/base/test_sim_backend.py::TestMuJoCoBasic tests/base/test_sim_backend.py::TestMuJoCoBodySensors tests/base/test_sim_backend.py::TestCrossBackend tests/base/test_sim_backend.py::TestCrossBackendBodySensors tests/base/test_sim_backend.py::test_motrix_backend_fixed_base_set_state_matches_mujoco_for_hand_and_ball tests/envs/test_allegro_domain_randomization.py::test_allegro_mujoco_reset_applies_base_mass_and_com_domain_randomization 'tests/envs/test_env_configs.py::test_env_reset_and_step[AllegroInhandRotation]' tests/envs/test_env_configs.py::test_go1_env_initializes_kp_kd_into_pool tests/envs/test_env_configs.py::test_go2_env_initializes_kp_kd_into_pool tests/envs/test_env_configs.py::test_allegro_env_initializes_kp_kd_into_pool -q
• UV_CACHE_DIR=/tmp/uv-cache make test-slow