Integrate dexsim gizmo controllers

[yuecideng]

Description

Summary

• Delegate world-level rigid-body and articulation manipulation to dexsim's EntityGizmoManipulator.
• Enable entity gizmos by default when a viewer window opens, keep headless simulations disabled, and expose explicit enable/disable/query APIs.
• Register EmbodiChain's built-in default_plane as a static external target so it cannot be manipulated.
• Bind robot TCP gizmos to dexsim's IKGizmoController and NewtonChainIK, while adapting joint state reads and writes through EmbodiChain's Robot abstraction.
• Update gizmo examples and documentation, and add lifecycle, exclusion, and robot-controller tests.

Dependency / merge gate

This PR requires the next dexsim release and must not be merged or released before that version is published and EmbodiChain's dexsim requirement is updated.

The required dexsim release must include:

• World.enable_entity_gizmo(), World.disable_entity_gizmo(), and World.get_entity_gizmo();
• multi-target EntityGizmoManipulator support, including register_external_target();
• dexsim.kit.ik.IKGizmoController and dexsim.kit.ik.NewtonChainIK.

The dependency file is intentionally unchanged because that dexsim version has not been released yet.

Related: #420

Type of change

• New feature (non-breaking change which adds functionality)
• Documentation update

Screenshots

Not applicable; behavior is covered by automated tests and interactive dexsim smoke tests.

Validation

• black .
• git diff --check
• python -m compileall on changed Python modules
• pytest -q tests/sim/test_cfg.py tests/sim/test_sim_manager.py tests/sim/objects/test_gizmo.py — 28 passed
• pytest -q tests/sim/objects/test_robot_cfg.py -k 'not pk_dof' — 20 passed
• pytest -q tests/sim/objects/test_robot_cfg.py -k 'pk_dof' — 8 passed
• Interactive smoke checks for entity attach/detach, window reopen/config restore, headless behavior, default-plane exclusion, and robot IK on CPU/CUDA

Checklist

• I have run the black . command to format the code base.
• I have made corresponding changes to the documentation.
• I have added tests that prove my feature works.
• Dependencies have been updated — pending the next dexsim release described above.

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