The fix for CVE-2026-54093 rewrote backslashes to the path separator "/" in
archive entry names. On POSIX hosts a backslash is a legal filename byte, so
that rewrite manufactured a traversal sequence ("..\..\x" -> "../../x") out
of a single in-scope file, turning a Windows-only zip-slip into a cross-platform
one. Neutralize backslashes to an inert character instead, and reject any entry
whose name is not already a normalized root-relative path. Adds a regression
test that downloads a folder containing a backslash-named file as a zip.
