Path.anchor
string anchor() const @safeReturn the path anchor (drive + root on Windows; root on POSIX).
Platform semantics:
- POSIX: anchor is "/" for absolute paths, empty otherwise.
- Windows:
- Drive absolute: "C:\\"; drive-relative: "C:".
- UNC: "\\\\server\\share\\" (trailing backslash by convention, matching pathlib).
- Root-only (no drive): "\\".
Returns
The anchor string, or empty if none.