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_grounded.py

@@ -148,7 +148,8 @@ def test_find_gaps_topic_with_no_chunks(
 ) -> None:
     """When retrieval returns no chunks, a gap lead is emitted directly."""
     monkeypatch.setattr("codex.synthesis._retrieve_chunks", lambda queries, top_k: [])
-    leads = find_gaps(llm=StubLLM(""))
+    # Coverage gaps are opt-in now (audit R-8): pass an explicit topic.
+    leads = find_gaps(llm=StubLLM(""), topics=["nonexistent topic"])
     assert len(leads) >= 1
     assert all(lead.kind == "gap" for lead in leads)
     # Direct gap leads carry a validation hint (re-ingest)