dirfs: reject paths that escape the root via ".."

[sahvx655-wq]

1. DirFileSystem is meant to keep every operation under its configured path, but _join only concatenates the prefix with the caller-supplied path and never inspects ".." segments.
2. a relative path such as "../secret" therefore resolves above the root, and since reads and writes both funnel through _join an untrusted name escapes the directory entirely (confirmed by reading and writing a file outside the root over LocalFileSystem).

Reject in _join when the path would climb above the root; ".." that stays within the prefix is still allowed, so existing in-tree paths are unaffected.