feat(graph): warn on low citing-paper coverage in graph report (R-D)

Add graph.citing_coverage() (papers with >=1 out-edge / total) and a graph_min_citing_coverage setting (default 0.8). codex graph report now surfaces 'Citing coverage: N/M papers with out-edges (P%)' in text and JSON, and warns when the share is below threshold -- low citing coverage starves PageRank/coupling even when the paper count looks healthy (the DQ-1 sub-item). Separate from the existing total-count graph_min_corpus_size warning.

Tests: citing_coverage unit tests + CLI tests (line shown, warning fires/suppressed, JSON field). Full suite green; ruff + mypy clean. Live: graph report shows 29/29 (100%), no warning.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

tests/graph/test_cli.py

@@ -117,6 +117,36 @@ class TestGraphReport:
         assert result.exit_code == 0
         assert "Warning" in result.output or "warning" in result.output
 
+    def test_shows_citing_coverage_line(self):
+        # _sample_graph: A, B, C all cite → 3/3 = 100%, so the line shows but no warning.
+        out = self._run(known_ids=("A", "B", "C")).output
+        assert "Citing coverage" in out
+        assert "weakens" not in out  # 100% coverage → no R-D warning
+
+    def test_low_citing_coverage_warning(self):
+        # Only A has an out-edge; B and C are ingested but cite nothing → 1/3 = 33%.
+        g = nx.DiGraph()
+        g.add_edge("A", "X")
+        g.add_nodes_from(["B", "C"])
+        result = self._run(graph=g, known_ids=("A", "B", "C"))
+        assert result.exit_code == 0
+        assert "weakens PageRank" in result.output  # the R-D coverage warning
+
+    def test_json_includes_citing_coverage(self):
+        import json
+
+        g = nx.DiGraph()
+        g.add_edge("A", "X")
+        g.add_nodes_from(["B", "C"])
+        conn = _make_conn_with_paper_ids("A", "B", "C")
+        with (
+            patch("codex.db.get_conn", side_effect=_make_conn_cm(conn)),
+            patch("codex.graph.build_citation_graph", return_value=g),
+        ):
+            result = runner.invoke(app, ["graph", "report", "--json"])
+        data = json.loads(result.output)
+        assert data["citing_coverage"] == {"citing": 1, "papers": 3, "fraction": 0.3333}
+
 
 # ---------------------------------------------------------------------------
 # codex graph related