Replace 3-gram substring match with:
- _last_sentence(): only the last sentence before a citation is checked
(prevents a hallucinated paragraph grounding via a phrase at its end)
- _content_words(): stopwords removed from both claim and chunk before
n-gram comparison (prevents bypass via "the discrete conformal map")
- Content-5-gram: require 5 consecutive non-stopword tokens from last
sentence to appear in the chunk's content-word stream
- Claims with <5 content words in last sentence → ungrounded by default
Add adversarial tests: stopword-3-gram bypass → grounded=False;
legitimate content-5-gram → grounded=True.
Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>
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