feat(backend): expose manager actuation metadata

[TATP-233]

Summary

• add formal cold-path SimBackend contracts for actuator names, actuator-target joint names, and default DoF positions
• implement the metadata in MuJoCo, Motrix, Drake, and mjwarp adapters without exposing backend-private model objects to env/manager code
• fail closed for non-joint/multi-DoF transmissions and for DrakeUni runtimes that do not expose real actuator names; no inferred-name fallback

Scope accounting

• 8 files changed: +292 / -0
• source-derived code: 0 lines
• mechanical Torch → NumPy adaptation: 0 lines
• UniLab-specific backend contract/adapter implementation: 196 additions
• focused/conformance tests: 96 additions
• modified existing: 8 files; new/deleted files: 0; deleted lines: 0

Validation

• focused and representative backend/entity/lifecycle regression: 74 passed, 15 skipped
• direct Go2 MuJoCo/Motrix metadata probe confirmed identical actuator/control ordering and target-joint mapping
• make test-all: 1,838 passed, 25 skipped, 1 xfailed; ruff, mypy, and pyright passed; benchmark smoke 65/67 with only two platform-optional mlx cases skipped
• Drake production dependency is not installed in this workspace; fake runtime contract tests cover valid metadata and explicit missing/invalid failures, without claiming an unexecuted production backend

Implements #1052
Umbrella: #1042