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>

codex/cli.py

@@ -422,7 +422,15 @@ def synthesis_leads(
         "--lib-path",
         help=(
             "Path to a C++ source tree. Enables improvement leads (@cite-aware) and "
-            "code-vs-corpus gap detection. Without it only connection + topic gaps run."
+            "code-vs-corpus gap detection."
+        ),
+    ),
+    topic: list[str] | None = typer.Option(  # noqa: B008
+        None,
+        "--topic",
+        help=(
+            "Topic to check for coverage gaps (repeatable). Opt-in: without --topic "
+            "no coverage-map gaps run (avoids per-paper false positives, audit R-8)."
         ),
     ),
     output_dir: Optional[str] = typer.Option(  # noqa: UP045
@@ -444,7 +452,7 @@ def synthesis_leads(
     llm = default_llm_client()
 
     connections = find_connections(top_k=settings.synthesis_top_k, llm=llm)
-    gaps = find_gaps(lib_path=lib_path, llm=llm)
+    gaps = find_gaps(lib_path=lib_path, llm=llm, topics=topic or None)
     improvements = (
         find_improvements(lib_path, top_k=settings.synthesis_top_k, llm=llm) if lib_path else []
     )