fix(wiki): sentence-level claim granularity + content-5-gram grounding guard

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>

tests/wiki/test_compile.py

@@ -170,6 +170,51 @@ def test_grounding_guard_ungrounded_when_bibkey_missing() -> None:
     assert result[0].grounded is False
 
 
+def test_grounding_guard_adversarial_stopword_bypass() -> None:
+    """Adversarial: stopword-3-gram matches are NOT sufficient for grounding.
+
+    'the discrete conformal map' appears in the chunk, but this is a stopword
+    phrase. The preceding hallucinated sentences contain NO content-5-gram
+    present in the chunk → grounded=False.
+    """
+    # The adversarial text from the session-prompt review finding:
+    adversarial_text = (
+        "The Riemann hypothesis is proven by combining Hodge theory with quantum entanglement. "
+        "Furthermore P=NP holds for hyperbolic tetrahedra. The discrete conformal map"
+    )
+    claim = Claim(
+        text=adversarial_text,
+        bibkey="Springborn2008",
+        locator="chunk 0",
+    )
+    result = _run_grounding_guard([claim], FIXTURE_CHUNKS)
+    assert result[0].grounded is False, (
+        "Stopword 3-gram 'the discrete conformal map' must NOT ground a hallucinated claim"
+    )
+
+
+def test_grounding_guard_last_sentence_grounded_earlier_hallucination_ignored() -> None:
+    """Only the last sentence is checked for grounding.
+
+    If the last sentence IS grounded, the earlier hallucinated sentence is
+    irrelevant (it has no citation → it won't be parsed as a Claim at all).
+    This test verifies that a claim whose text = the last sentence grounds correctly.
+    """
+    # The last sentence of the claim text is directly grounded in the fixture chunk.
+    grounded_sentence = (
+        "the hyperbolic volume is V = L(gamma1) + L(gamma2) + L(gamma3)"
+    )
+    claim = Claim(
+        text=grounded_sentence,
+        bibkey="Springborn2008",
+        locator="chunk 16",
+    )
+    result = _run_grounding_guard([claim], FIXTURE_CHUNKS)
+    assert result[0].grounded is True, (
+        "Last sentence with content-5-gram present in chunk must be grounded"
+    )
+
+
 # ---------------------------------------------------------------------------
 # _inject_cross_refs
 # ---------------------------------------------------------------------------