feat(motrix): document and test cpu_ids worker affinity wiring (#962)

[TATP-233]

Summary

Closes the UniLab side of #962 (Motrix worker CPU affinity). The upstream pieces are already delivered and locked in this repo:

• motrixsim-core==0.10.1 (uv.lock) exposes motrixsim.init_thread_pool(core_ids=...).
• unisim-core==1.7.4 (uv.lock; feat: add Motrix worker CPU affinity and upgrade motrixsim-core to 0.10.1 unilabsim/unisim#235) implements the Motrix adapter: cold-path cpu_ids validation mirroring the MuJoCo BatchEnvPool contract, init_thread_pool pinning before the first model load, and a read-only backend.cpu_ids property.
• UniLab already forwards EnvCfg.cpu_ids to every backend via env_backend_kwargs (src/unilab/base/backend_factory.py) and confines the env process via apply_env_cpu_runtime (src/unilab/base/cpu_runtime.py).

What was missing on the UniLab side, delivered here:

• EnvCfg.cpu_ids docstring generalized beyond MuJoCo to document the Motrix worker-pool pinning (src/unilab/base/base.py).
• New tests/base/backend/test_motrix_cpu_affinity_wiring.py mirroring the MuJoCo wiring tests: env_backend_kwargs mapping, create_backend routing (direct kwargs and via EnvCfg), cold-path rejection of empty/duplicate/negative/unavailable ids, and the default path leaving the pool untouched. motrixsim.init_thread_pool is mocked because the pool is process-wide and initializable only once; the behavioral per-thread pinning check lives in unisim (tests/adapters/motrix/test_cpu_ids.py, perf(motrix): route get_body_state_w through fused get_link_states + affinity behavioral test unilabsim/unisim#288).
• Motrix backend user-guide pages (en + zh_CN) document the affinity option.

No hot-path changes: affinity is only validated/applied during backend construction.

Validation

Commands run on the final head:

uv run --no-sync pytest tests/base/backend/test_motrix_cpu_affinity_wiring.py -q
uv run --no-sync pytest tests/base/backend/ tests/base/test_motrix_backend_options.py -q
make check
uv run --no-sync pytest tests/scripts/test_check_docs.py -q
cd docs/sphinx && UNILAB_DOCS_SKIP_AUTODOC=1 uv run --no-project --with-requirements requirements.txt sphinx-build -b html -n source build/html
make test-all

Results:

• wiring tests: 8 passed
• make check: ruff/mypy/pyright clean (2 pre-existing viser import warnings)
• doc checks: 19 passed; sphinx build succeeded with no new warnings
• make test-all: exit 0 — test-cov suite passed (1518 passed, 28 skipped after pulling the allegro_hand asset pack; the 2 initial smoke failures were missing local meshes, not code) and benchmark smoke 35/35 module-mode + 36/36 script-mode

Impact

• Backend impact: Motrix adapter wiring tests + docs only; runtime behavior unchanged (the affinity path already ships in unisim-core 1.7.4)
• Platform impact: Linux (affinity); validation errors are platform-independent
• Training effect expected: none

Linked work

• Closes feat(motrix): 为 motrixsim 后端增加显式 CPU affinity #962
• Related: feat(mujoco): 为 BatchEnvPool 增加显式 CPU affinity #959 (MuJoCo BatchEnvPool.cpu_ids), feat: Motrix worker CPU affinity and motrixsim-core 0.10.1 upgrade unilabsim/unisim#234 (upstream adapter), perf(motrix): route get_body_state_w through fused get_link_states + affinity behavioral test unilabsim/unisim#288 (behavioral pinning test)