docs: define NumPy manager API migration contract

[TATP-233]

Summary

• pin mjlab v1.6.0 commit 0fb8a681 as the source-aligned manager API baseline
• define canonical manager/config semantics and documented NumPy intentional differences
• resolve Work: 评估面向场景和任务 API 的 entity abstraction #586 with a minimal base-owned scene/entity facade boundary
• require fail-closed capabilities, provenance accounting, semantic-first performance policy, and eventual removal of the legacy task path

This PR contains contract documentation only; it does not implement manager runtime, backend capability, lifecycle, task, runner, or IPC changes.

Scope

• 8 documentation/config files
• 316 inserted lines
• source-derived code: 0
• mechanical conversion: 0
• UniLab-specific runtime glue: 0
• deleted runtime code: 0

Validation

• uv run pytest tests/scripts/test_check_docs.py -q — 20 passed
• uv run pytest tests/scripts/test_repo_hygiene.py -q — 6 passed
• UNILAB_DOCS_SKIP_AUTODOC=1 uv run --no-project --with-requirements requirements.txt sphinx-build -b html -n source build/html — passed
• make test-all — 1,769 passed, 24 skipped, 273 deselected, 1 xfailed; benchmark smoke 32/33 module-mode and 33/34 script-mode with only optional MLX skips

Impact

No runtime or user behavior changes. MuJoCo, Motrix, Drake, macOS, Linux, training, and IPC paths are unchanged.

Refs #1043
Umbrella: #1042
Related: #586