docs: validate Windows PRs via Parallels+SSH; gitignore .parallels

Document the Mac-host -> Parallels Windows 11 SSH workflow for validating
Windows-specific behavior, the win32-gated test convention (it.runIf), and
guest toolchain quirks (PATH refresh, Windows-local clone, VC++ ARM64 redist).

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>