GHSA-568f-pv23-39p4: NLTK: Stable FrameNet and NKJP readers parse outside-root XML

### Summary Published `nltk==3.9.4` still contains several XML-reader entrypoints that build parser paths from caller-controlled selectors or trusted-looking index state without preserving the corpus-root boundary. ### Details - **Vulnerability type:** Path traversal and trusted-root bypass - **Affected component:** `FramenetCorpusReader.frame_by_name`, `FramenetCorpusReader.doc`, `FramenetCorpusReader.lu`, `NKJPCorpusReader.header` - **Affected versions:** Published `3.9.4` reproduced. Current source `v3.10.0-rc2` acted as a negative control and blocked the same payloads. …