Add Windows validation guidance and preserve boxed booleans

[dmitriyborodko]

Summary

• Document successful Windows x64 Debug and Release validation and the Swift toolchain workarounds.
• Make CoreFoundation optional and preserve boxed Foundation booleans as distinct from numeric zero and one.
• Add a serialization contract test and update the tested-platform badge and DocC guide.
• Allow manual PR CI on macOS, Linux, and Windows by adding the run-ci label. PR creation and new commits do not start CI; remove and reapply the label to check new changes. Other labels do not cancel a requested run.

Testing

• Functional suite passed locally on Windows x64 with Swift 6.4.1 in Debug and Release, excluding performance tests.
• Validated workflow YAML, label conditions for both test jobs, concurrency isolation, and git diff --check.