Support Windows as a shipped platform

[fahedouch]

Separate from #240 (which just keeps the build green). This is the bigger commitment: actually ship and support Windows.

What it takes:

• windows/amd64 (arm64?) in the release matrix + SHA256SUMS.
• Run the tests on windows-latest, not just build. os.Rename over an existing file behaves differently on Windows (the atomic-write in fix(generate): write output atomically via temp file + rename #237), so the rename path needs real testing there.
• Handle install in vens-action (linux/darwin only today).
• Install docs.

Worth weighing before doing it: vens is a CI/container tool and that world is mostly Linux, so the demand isn't obvious, and once we ship it we own its bugs. Depends on #240.