Extends the CWE-22 path traversal guard to cover Windows absolute paths
of the form C:/... and D:\... — previously only leading / and \ were
checked, which missed drive-letter prefixes. Replaces the inline
startswith check with a compiled module-level regex (_TRAVERSAL_RE) that
covers all three attack patterns: .., leading /\, and leading X: drives.
Adds two regression tests for C:/windows/system32 and D:\\path\\to\\file.
Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>