Add Windows process-tree fixture validation

[itscloud0]

Problem

Windows process-tree termination is represented by the windows-tree-termination fixture, but release validation currently skips it on macOS/Linux.

Useful outcome

Add a Windows CI job or documented manual validation path that runs the Windows fixture and records whether owned child processes are terminated correctly after timeout.

Acceptance criteria

• Windows fixture runs on a Windows host in CI or a documented reproducible manual check.
• The report distinguishes PASS, FAIL, and UNSUPPORTED clearly.
• README/validation docs state the Windows support status without implying untested coverage.