epath.Path.replace() silently destroys the destination file on Windows when the source is a directory

`_OsPathBackend.replace()` guards only the destination:

```python
def replace(self, path: PathLike, dst: PathLike) -> None:
if self.isdir(dst):
raise IsADirectoryError(f'Cannot overwrite: {dst} is a directory')
os.replace(path, dst)
```

Nothing checks for *source is a directory, destination is an existing file*. On Windows `os.replace` accepts that, and the destination file's contents are gone.

```python
# Windows 11, CPython 3.13.13
os.mkdir(srcdir)
open(os.path.join(srcdir, 'payload.txt'), 'w').write('inside the source dir')
open(dst, 'w').write('IRREPLACEABLE DESTINATION CONTENT')

os.replace(srcdir, dst) # returns, no exception
os.path.isdir(dst) # True
os.listdir(dst) # ['payload.txt'] - original bytes gone
```

On POSIX `rename(2)` returns `ENOTDIR` itself, so the missing guard never mattered there, and `pytest_and_autopublish.yml` runs `ubuntu-latest` only.

The sibling `_FileSystemSpecBackend.replace()` already has the check at `backend.py:548-551`, so the two backends currently disagree about the same `Backend` contract.

`test_backend[_test_replace]` already asserts the correct behaviour and reports `SUBFAILED[os/dst-folder0-file.txt] - Failed: DID NOT RAISE NotADirectoryError`. It has never run on Windows, because the module fails collection there on `import grp`.

Two things I hit attempting a patch, which is why this is an issue and not a PR:

`exists` and `isdir` delegate straight to `os.path.exists` and `os.path.isdir` (`backend.py:135-139`), and both dereference symlinks where `rename(2)` does not. Copying the sibling's guard verbatim would raise for a symlink-to-directory source, which POSIX permits. Adding `not os.path.islink(path)` restores that, but Windows directory symlinks carry `FILE_ATTRIBUTE_DIRECTORY` on the link entry itself, so I could not establish the carve-out is safe on the platform the guard exists for. Creating a symlink here needs a privilege this account lacks (WinError 1314).

Separately, `backend.py:217-218` raises for any directory `dst`, while `rename(2)` succeeds onto an empty one.

I have no Linux or macOS machine, so every POSIX statement above is read from the contract rather than measured.

Start in backend.py at _OsPathBackend.replace(), then compare its behavior with _FileSystemSpecBackend.replace() at lines 548-551. Run test_backend[_test_replace] on Windows after addressing the import grp collection failure, and verify the source-directory/destination-file case without breaking the documented symlink and destination-directory behavior.

Written by the indexing model from the issue text.