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_graph.py

@@ -10,6 +10,7 @@ import pytest
 from codex.graph import (
     build_citation_graph,
     citation_pagerank,
+    citing_coverage,
     dangling_citations,
     find_co_cited,
     find_related,
@@ -287,3 +288,24 @@ class TestDanglingCitations:
         g = build_citation_graph(_make_conn(rows))
         dangling = dangling_citations(g, set())
         assert set(dangling) == {"A", "B"}
+
+
+# ---------------------------------------------------------------------------
+# citing_coverage (R-D)
+# ---------------------------------------------------------------------------
+
+
+class TestCitingCoverage:
+    def test_counts_papers_with_out_edges(self):
+        # _small_graph: A→.., B→.., C→D have out-edges; D, E have none.
+        g = _small_graph()
+        n_citing, n_papers = citing_coverage(g, {"A", "B", "C", "D", "E"})
+        assert (n_citing, n_papers) == (3, 5)
+
+    def test_paper_absent_from_graph_counts_as_no_out_edges(self):
+        # A cites; Z was ingested but has no chunks/edges, so it isn't a graph node.
+        g = _small_graph()
+        assert citing_coverage(g, {"A", "Z"}) == (1, 2)
+
+    def test_empty(self):
+        assert citing_coverage(nx.DiGraph(), set()) == (0, 0)