CVE-2026-34726: Copier `_subdirectory` allows template root escape via parent-directory traversal

Copier’s _subdirectory setting is documented as the subdirectory to use as the template root. However, the current implementation accepts parent-directory traversal such as .. and uses it directly when selecting the template root.

As a result, a template can escape its own directory and make Copier render files from the parent directory without --UNSAFE.