Add headless window capture API

[yuecideng]

Description

Adds SimulationManager.capture_window() for single-frame RGB capture using the same hidden-camera approach as viewer window recording. When a viewer window is open, the capture camera follows the current viewer pose; in headless runs it renders from a provided pose or a default look-at pose.

Also updates scripts/tutorials/sim/create_scene.py with --capture-window, --capture-path, and --max-steps so the feature can be verified non-interactively.

Fixes #

Type of change

• New feature (non-breaking change which adds functionality)
• Enhancement (non-breaking change which improves an existing functionality)

Screenshots

Not applicable. Verification writes ./outputs/window_capture/create_scene_verify.png locally.

Verification

• python -m py_compile embodichain/lab/sim/sim_manager.py scripts/tutorials/sim/create_scene.py
• black --check embodichain/lab/sim/sim_manager.py scripts/tutorials/sim/create_scene.py
• python scripts/tutorials/sim/create_scene.py --headless --max-steps 5 --capture-window --capture-path ./outputs/window_capture/create_scene_verify.png
• Verified generated image is 1920x1080 RGB and nonblank.

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 fix is effective or that my feature works
• Dependencies have been updated, if applicable.