`pathdiff::diff_paths` does not return `None` when the two paths sit on
different Windows drive roots; it produces a path that climbs out with
`..` and then re-anchors with the target's drive letter (for example,
`..\..\C:\Users\...`). Windows rejects such a symlink target with
`ERROR_INVALID_PARAMETER` (os error 87), which is the failure mode CI
on Windows hits when the workspace lives on `D:` and the global store
installed by `setup-pnpm` lives on `C:`.

Node.js's `path.win32.relative` returns the absolute `to` argument
when the two paths share no common root, which is the behavior
upstream `symlink-dir` relies on. Mirror that by detecting unequal
`Component::Prefix` values up front and returning the absolute target
unchanged. Drive-letter comparison is case-insensitive to match
NTFS and Node.js semantics.

`Prefix::Disk('C')` and `Prefix::VerbatimDisk('C')` name the same
physical drive, but compare unequal as `Component::Prefix` values, so
the cross-root guard added in the previous commit would unnecessarily
fall back to an absolute target whenever one input was a verbatim
`\\?\C:\...` path. Worse, `pathdiff::diff_paths` itself would emit a
re-anchored garbage path on the variant-mismatched case if the guard
ever let it through.

Run both inputs through `dunce::simplified` on Windows, which strips
the `\\?\` prefix when the non-verbatim form is still valid, so
mixed verbatim and non-verbatim views of the same drive collapse
onto a common `Prefix::Disk` before the comparison and the diff.
Tighten `case_normalize` to map `VerbatimDisk` onto `Disk` for the
residual long-path case that `dunce` declines to simplify.

`clippy::needless_return` rejects the explicit `return` in the
Windows `{ ... }` block because the block itself is the function's
tail expression on Windows. The block-with-cfg-attribute layout
also obscured that the function body shrinks to a single tail
expression on non-Windows. Extracting two `#[cfg]`-gated
`relative_target_inner` helpers makes both targets read as a
straight tail expression and drops the clippy violation.

Collapsing `Disk` and `VerbatimDisk` (and the UNC analogues) onto a
common variant in `same_path_root` is unsafe when one input is a
verbatim path that `dunce::simplified` declined to strip (e.g., a
path long enough that the non-verbatim form would exceed the legacy
limit). `same_path_root` would declare the two paths same-root, but
`pathdiff::diff_paths` would still walk `Component::Prefix` for
equality and emit a re-anchored garbage path on the variant
mismatch — the same `..\\..\\C:\\Users\\...` shape that triggers
`ERROR_INVALID_PARAMETER` (os error 87) on Windows.

Restore the variant-strict comparison so mismatched variants fall
back to an absolute target, which is the correct outcome on the
edge case. The common short-path mixed-form case (`\\?\C:\foo`
paired with `C:\bar`) still produces a relative target because
`dunce::simplified` normalizes the variants upstream of this check.

The previous doc claimed "same UNC share" as a match condition, but
the implementation requires identical `Prefix` after `dunce::simplified`
with only drive-letter case folding. UNC server/share differences in
case or `UNC`/`VerbatimUNC` variant fall back to absolute.