feat(env): add named sensor observation terms

[TATP-233]

主要结果

把已完成的 backend named-sensor contract 接到 Manager-Based scene/context：builtin_sensor 与 projected_gravity_from_sensor 在 term construction 冷路径绑定 BackendSensorView，热路径只读取 backend-owned view。

• EntityScene.bind_sensor_data(...) 只委托正式 SimBackend contract，不暴露 backend/model/data。
• 保留 pinned mjlab term 名称、sensor_name 参数与 projected-gravity 取负语义。
• unknown/duplicate/空名称、非 3-D gravity 以及 runtime shape/dtype/finite drift 均 fail-closed，并携带 term/backend 诊断。
• 不新增 SceneCfg.sensors、backend 方法或 sensor composer；不迁移 production task、runner、IPC 或 scripts。

Validation

• make test-all
• ruff format/check、mypy、pyright
• 1978 passed, 27 skipped, 276 deselected, 1 xfailed
• focused owner-boundary suite：189 passed, 4 skipped, 4 deselected
• focused sensor observation suite：25 passed
• benchmark smoke：module mode 32/33、script mode 33/34（各 1 个 platform-optional MLX skip）

Change accounting

• source-derived public names/equations: 约 8 行
• mechanical Torch→NumPy conversion: 0 行
• UniLab-specific scene/protocol/class-term glue: 约 108 行
• tests/docs: 约 156 行
• reworked/deleted: 2 行；0 files
• 7 files changed; 272 additions, 2 deletions

Closes #1078
Related to #1042