feat(env): add manager pd gain reset event

[TATP-233]

Summary

• 新增 NumPy mdp.pd_gains reset event，对齐 pinned mjlab 的 selector、uniform/log_uniform、scale/abs 与逐 actuator 独立采样语义
• 在 Entity 冷路径完成 local actuator selector 到 backend column 的映射与 capability/default gain 绑定
• 将 kp/kd 与 qpos/qvel 合并为 exactly-once SimBackend.set_state payload；稀疏 row、unsupported mode/backend 均 fail-closed
• 增加 fake transaction/event evidence 及真实 MuJoCo/Motrix reset mutation evidence

Refs #1092
Umbrella #1042

Scope

• 7 files
• 650 net handwritten lines
• no runner, IPC, legacy DR, production Go2 config, or new backend method changes

Validation

• make test-all
  • 2019 passed
  • 27 skipped
  • 276 deselected
  • 1 xfailed
  • ruff, mypy, pyright, coverage suite, benchmark smoke all passed
• Remote CI intentionally skipped per the approved Roadmap: 将 UniLab 迁移为社区通用的 Manager-Based API #1042 development strategy; local full gate is authoritative.