feat(issue-1509): wire Newton backend owner

[TATP-233]

Summary

• Wire UniLab's EnvCfg, registry, CLI, backend factory, and process-device boundary to the optional UniSim Newton adapter.
• Add G1 walk-flat Newton owners for PPO and SAC with explicit capacity settings (320 contacts / 512 constraints, calibrated against measured broadphase peaks of ~258/269) and fail-closed PD randomization.
• Add bilingual Newton backend docs (installation via the isolated newton extra, conflict declarations vs mujoco/mjwarp extras, record playback) and installation-guide entries.
• Add sim2sim audit/support-matrix coverage and optional-runtime/import/conformance tests; SAC Newton is promoted to Tested after a full real-hardware training + playback validation.
• Keep legacy unisim-core compatibility by stripping Newton-only kwargs for non-Newton backends.

Parent / dependency

• Parent roadmap: roadmap: Newton 物理后端接入（承接 UniLab#1338，适配 unisim 拆分后架构） unilabsim/unisim#22
• Dependencies: child: Newton contact sensor 与 record playback 支持（#22） unilabsim/unisim#31 (UniSim PR #32, Newton contact sensor + record playback + reset/audit fixes) and unilab-rl PR (process-device binding for newton in spawn collectors) — both required to reproduce the training run below.
• Child issue: child: Newton backend owner wiring and task smoke test (#22) #1509

Validation

• make test-all on final head 470a6433 — passed (full suite + benchmark smoke; 1 platform-optional mlx skip).
• Full SAC Newton training on real hardware (RTX 4090, torch 2.8.0+cu128, newton 1.5.1, mujoco-warp 3.11): 5000/5000 iterations, reward/mean 6.68 → 242.3, episode length → 983/1000, ~43k steps/s, 4m25s wall (run logs/fast_sac/G1WalkFlat/2026-09-06_01-21-36_newton/); checkpoints model_1000–5000.pt, ONNX export verified (max_diff 7.6e-07), and record playback on model_5000.pt (800-frame MuJoCo snapshot render, walking gait confirmed). Recorded in _MAINTAINER_VALIDATED_NEWTON_ENTRYPOINT_TASKS.
• uv run scripts/audit_sim2sim_contracts.py — G1 PPO/SAC MuJoCo↔Newton TRANSFERABLE.
• pytest tests/scripts/test_support_matrix.py -m slow (12 passed), owner-contract -k newton (2 passed), tests/scripts/test_check_docs.py (20 passed), Sphinx build clean.
• Newton runtime/CUDA is not installed in this CI environment; Newton conformance/import runtime tests fail closed or skip with actionable diagnostics.

Notes

The Newton extra is isolated from the historical mjwarp/mujoco extras through uv conflict declarations and pins Newton 1.5.1 with MuJoCo-Warp 3.11 / Warp 1.16 (trimesh added for the MJCF importer cold path). PPO Newton remains Configured without runtime evidence.