fix(synthesis): make coverage-map gaps opt-in (audit R-8)

find_gaps defaulted its probe topics to paper titles, so a title — which
retrieves mostly its own single bibkey — fell below synthesis_gap_min_coverage
and flagged almost every paper as a 'gap'. Coverage-map gaps now run only for
explicitly-requested topics; the CLI gains a repeatable --topic option. The
code-vs-corpus gap path (precise) is unchanged and still default.

Tests: the two cases that relied on default-title coverage gaps now pass explicit
topics; added a regression that no coverage gaps appear without --topic.

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>

tests/synthesis/test_gaps.py

@@ -112,10 +112,35 @@ def test_find_gaps_coverage_map_low_coverage_triggers_llm(
         "For an ideal tetrahedron with dihedral angles the hyperbolic volume is "
         "V = L(gamma1) + L(gamma2) + L(gamma3). [Springborn2008 #chunk 16]\n"
     )
-    leads = find_gaps(llm=StubLLM(grounded_body))
+    # Coverage gaps are opt-in (audit R-8): pass the topic explicitly.
+    leads = find_gaps(llm=StubLLM(grounded_body), topics=["hyperbolic volume formula"])
     assert any(lead.kind == "gap" for lead in leads)
 
 
+def test_find_gaps_no_coverage_gaps_without_explicit_topics(
+    monkeypatch: pytest.MonkeyPatch,
+    mock_paper_titles: None,
+    mock_settings: Path,
+) -> None:
+    """Without explicit topics, paper titles do NOT auto-generate coverage gaps (R-8)."""
+    sparse_chunks: list[dict[str, Any]] = [
+        {
+            "id": 10,
+            "paper_id": "springborn-2008",
+            "ord": 16,
+            "content": "the hyperbolic volume is V = L(gamma1) + L(gamma2) + L(gamma3)",
+            "bibkey": "Springborn2008",
+        },
+    ]
+    monkeypatch.setattr(
+        "codex.synthesis._retrieve_chunks",
+        lambda queries, top_k: [dict(c) for c in sparse_chunks],
+    )
+    # No lib_path, no topics → the coverage map must not run despite sparse coverage.
+    leads = find_gaps(llm=StubLLM("a gap. [Springborn2008 #chunk 16]"))
+    assert leads == []
+
+
 def test_find_gaps_explicit_topics_override_default(
     monkeypatch: pytest.MonkeyPatch,
     mock_paper_titles: None,