Handle filesystem roots in DirFileSystem path translation

[zhechen-cs]

DirFileSystem rooted at a local filesystem root can raise AssertionError when listing that root. Root path joining and relative-name stripping both add a separator to a prefix that already ends with one; POSIX double-leading-slash normalization can also make leading-slash and ordinary relative queries disagree.

Join root prefixes with one separator and use the same prefix when returning relative paths. Preserve existing subtree behavior and trailing-slash copy semantics. Add POSIX/drive-root path cases for synchronous/asynchronous wrappers and real local ls(), info(), find() and glob() checks with both forms of the query path.

Validation on macOS arm64 / CPython 3.12.13, independently based on upstream 8cf902e:

• Unchanged-source path/regression selection: 10 failed, 22 passed.
• DirFS/local/core/spec/mapping/chained suites: 801 passed, 92 skipped, 1 xfailed. The clean baseline passes 785 tests with the same skips and expected failure.
• Direct local root listings for both empty and slash paths now succeed and return relative names.
• Applicable automatic pre-commit hooks and git diff --check pass.

The suite emits the same multiprocessing fork deprecation warning on the baseline and patch. Native Windows, cloud, FUSE and downstream matrices were not run locally. A changelog entry is included.

[martindurant]

Isn't this wrong? Since we join on sep, shouldn't path always be lstrip-ped?