Add native window single-frame capture

[yuecideng]

Description

This PR adds default native-window single-frame capture support to SimulationManager.

• Adds WindowCaptureCfg with an enabled-by-default C hotkey, configurable key, output path, and file prefix.
• Adds SimulationManager.capture_window() and capture_window_image() with owned NumPy image results and PNG export.
• Uses DexSim Windows.get_picture() when a native frame is available.
• Falls back to an offscreen camera at the current native-window pose for renderers without CPU swapchain readback, including the DexSim 0.5.0 Hybrid and FastRT paths.
• Rejects closed windows and empty frames without creating empty output files.
• Documents the configuration and updates focused manager/config tests.

This is a documented EmbodiChain fallback for DexSim issue #231; no DexSim dependency version change is required.

Fixes #692

Type of change

• Bug fix (non-breaking change which fixes an issue)
• Enhancement (non-breaking change which improves an existing functionality)
• Breaking change (existing functionality will not work without user modification)
• Documentation update

Validation

• black .
• python docs/scripts/check_api_docs.py
• python .agents/skills/project-dev-context/scripts/context.py check
• python -m pytest -q -c /dev/null --noconftest tests/test_agent_context_map.py tests/test_agent_context_tools.py (40 passed)
• Manual DexSim 0.5.0 Vulkan smoke checks for hybrid and fast-rt: native get_picture() returned an empty (0,) frame, while the current-pose offscreen fallback returned a non-empty (120, 160, 4) image.
• Focused EmbodiChain simulation tests were not runnable in the current host because the system Python is 3.13 while the installed DexSim native package is CPython 3.11; changed files were syntax-compiled and the pure-Python capture harness passed.

Screenshots

Not applicable; the change is a simulation API and native-window control.

Checklist

• I have run the black . command to format the code base.
• I reviewed affected documentation and agent context, updated them where needed, or explained why no update was needed.
• Public API changes are reflected in the API docs (python docs/scripts/check_api_docs.py), if applicable
• I have added tests that prove my fix is effective or that my feature works
• Dependencies have been updated, if applicable.