[TATP-233]

Summary

• replace hardcoded Motrix set_state quaternion/control offsets with metadata-driven floating-base and joint index mapping
• support fixed-base robot base pose/velocity reads via the base link when the embodiment has no floating base
• add Allegro fixed-base regression coverage alongside existing floating-base backend tests

Impact Scope

• Motrix backend state restore for floating-base and fixed-base embodiments
• fixed-base scenes with non-robot freejoints such as Allegro in-hand rotation
• backend regression coverage in tests/base/test_sim_backend.py

Validation

• UV_CACHE_DIR=.uv-cache make check
• UV_CACHE_DIR=.uv-cache uv run pytest tests/base/test_sim_backend.py -m slow -k "motrix_backend_fixed_base or TestMotrixBasic or TestCrossBackend" -q
• UV_CACHE_DIR=.uv-cache uv run pyright tests/base/test_sim_backend.py
• UV_CACHE_DIR=.uv-cache uv run mypy tests/base/test_sim_backend.py

Notes

• UV_CACHE_DIR=.uv-cache make test aborts during collection in tests/algos/test_mlx_ppo.py because the local MLX import crashes before test execution; this is environmental and unrelated to the Motrix backend change.

Fixes #226