fix: allow explicit SDK discovery outside Windows

[sit-d]

Summary

• Load the Windows registry module only when registry discovery is needed on Windows.
• Keep WINSDK_ROOT discovery and the Python entrypoint help available on other platforms.
• Preserve the existing registry fallback for ordinary Windows use.

Verification

• python3 -m py_compile tools/win32_scraper.py tools/psdk_common.py tools/psdk_function.py tools/psdk_interface.py tools/psdk_struct.py
• python3 tools/win32_scraper.py --help
• python3 tools/psdk_function.py --help
• python3 tools/psdk_interface.py --help
• python3 tools/psdk_struct.py --help
• A synthetic WINSDK_ROOT tree with two SDK versions selected the newest version.
• git diff --check HEAD~1..HEAD

Notes / Limitations

• Windows registry discovery was not exercised; that branch retains the same keys and lookup order.