fix(envs): move device cfg field onto base ManagerBasedEnvCfg

[KraHsu]

Problem

genelab play GeneLab-Rubiks-Play-v0 and GeneLab-Wuji-Hand-Playback-v0 both crash at env construction:

AttributeError: 'RubiksEnvCfg' object has no attribute 'device'
AttributeError: 'WujiEnvCfg' object has no attribute 'device'

Root cause

• ManagerBasedRlEnv.__init__ reads cfg.device (src/genelab/envs/manager_based_rl_env.py:97,104).
• device: str = "cuda" was defined only on the RL subclass ManagerBasedRlEnvCfg, not on the base ManagerBasedEnvCfg.
• The Rubiks/Wuji play scenes use base cfgs (RubiksEnvCfg(ManagerBasedEnvCfg), WujiEnvCfg(ManagerBasedEnvCfg)) but play_task → build_env wraps them in ManagerBasedRlEnv, which requires cfg.device.

Fix

• Add device: str = "cuda" to the base ManagerBasedEnvCfg (src/genelab/configs.py) so every env cfg carries it.
• Drop the now-redundant duplicate on ManagerBasedRlEnvCfg.

Reference audit

mcp__serena__find_referencing_symbols on ManagerBasedRlEnvCfg/device:

src/genelab/envs/manager_based_rl_env.py:
  ManagerBasedRlEnv/_device  (line 96):  self._device = cfg.device
  ManagerBasedRlEnv/_scene   (line 103): InteractiveScene(..., device_hint=cfg.device)

A repo-wide grep for .device confirms every other access is on the runtime env.device property (managers, sensors, RL backends), never on a cfg object. Nothing relied on device being subclass-only, so the move is safe.

Tests

Headless regression tests (the AttributeError fires before scene build, so no GPU/sim needed):

• tests/test_configs.py — base ManagerBasedEnvCfg has a device field defaulting to "cuda".
• tests/test_examples_rubiks.py — RubiksEnvCfg().device == "cuda".
• tests/test_examples_wuji.py — WujiEnvCfg().device == "cuda".

Full suite: uv run pytest -q → 502 passed, 0 failures (headless).

Full play verification needs a GPU (cold Genesis kernel compile ~10 min); the unit tests above are the fast gate.

Docs

Moved device from the ManagerBasedRlEnvCfg row to the ManagerBasedEnvCfg row in docs/reference/configuration.{en,zh}.md.