Add Windows CI coverage and support Windows filesystem paths

[dmitriyborodko]

Summary

• Add opt-in Windows x64 Debug and Release checks, triggered by applying the run-ci pull request label.
• Support Windows drive and network paths in Codec.PathURL, preserving directory separators and literal path characters.
• Keep boxed Foundation booleans distinct from numeric zero and one on platforms without CoreFoundation.
• Update platform guidance, serialization notes, and the README's library comparison.

Testing

• Add contract tests for Foundation booleans and PathURL round trips, including Windows-specific paths.
• Windows x64 functional tests passed in Debug and Release with Swift 6.4.1; CI runs both configurations with documented toolchain workarounds.