fix: replace os.uname() with platform.node() for Windows compatibility

[Neon246760]

Description

os.uname() is a Unix-only system call. Running structchunk on Windows raises:
AttributeError: module 'os' has no attribute 'uname'

Related Issue

N/A

Type of Change

Please check the relevant options:

• Bug fix (non-breaking change that fixes an issue)
• New feature (non-breaking change that adds functionality)
• Breaking change (fix or feature that would cause existing functionality to change)
• Documentation update
• Refactor (no functional change)

Testing

Describe the tests you ran:

• Added new tests for the change
• All 192 existing tests still pass (pytest -q)
• Manually tested the change

Documentation

If this PR changes user-facing behavior:

• Updated the English canonical docs in README.md or docs/
• Updated the Chinese mirror in README.zh-CN.md or docs/zh-CN/
• Ran python scripts/check_doc_sync.py and confirmed 0 discrepancies

Checklist

• My code follows the project's style (PEP 8, type hints, docstrings)
• I have run pytest -q and all tests pass
• I have updated relevant documentation
• My changes do not introduce new warnings