CVE-2026-70626: NLTK: Symlink escape in CorpusReader allows arbitrary local file read outside the corpus root

nltk.corpus.reader.api.CorpusReader.open() can be used to read files outside the intended corpus root via a symlink placed inside that root. Although NLTK blocks absolute paths and .. traversal, the current boundary check is only lexical and does not account for symlink resolution. This leads to an arbitrary local file read / filesystem sandbox bypass for applications that rely on CorpusReader or FileSystemPathPointer to restrict file access.