On Windows, `D:` without a trailing backslash is a relative path
meaning 'current directory on drive D', not the drive root. This caused
tempdirs to be created inside `D:\uv` (the repo), where uv discovers
`.python-versions` and requests Python 3.14.0.

Move the Windows copy-on-write default from unconditional to gated behind
the `reflink-windows` preview feature flag. On Windows, the default link
mode remains `Hardlink` unless the user opts in via `--preview-feature
reflink-windows` or `--preview`.

When the preview flag is enabled, the effective link mode is overridden
from `Hardlink` to `Clone` at each command entry point, so that
copy-on-write is attempted on ReFS/Dev Drive volumes.