Arbitrary file overwrite via preserved directory symlinks

Symlink following in Copier before 9.11.2 allows context-dependent attackers to overwrite arbitrary files via a malicious template. When _preserve_symlinks is true, _render_template creates a directory symlink and then writes a rendered child path through it without confirming that the resolved destination remains under the output directory. The victim must render the template, success depends on os.scandir entry order, and writes are limited to files permitted by the victim's account.