Add coverage for router-center seeding, geometry batch reader, material topN cache, and setup-cache corruption paths

Closes the highest-value coverage gaps found via pytest-cov: pipeline.py's
EnergyRouter quantile-seeding (the roadmap's flagged fix for the failed MoE
rollout benchmark) had zero coverage, geometry.py's real parquet-batch reader
was always mocked, the material top-N-map cache-hit branch was untested
(only pdg's was), and setup_cache.py was missing malformed-cache-body and
unknown-axis error paths.

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

tests/test_setup_cache.py

@@ -128,6 +128,11 @@ def test_save_load_round_trip_topn_maps(tmp_path):
     assert mat_m.class_map == {"G4_AIR": 0, "PbWO4": 1}
 
 
+def test_topn_key_unknown_axis_raises():
+    with pytest.raises(ValueError, match="unknown top-N map axis"):
+        setup_cache.topn_key("process", 4)
+
+
 def test_load_missing_sidecar_returns_none(tmp_path):
     data = _touch_parquet(tmp_path / "shard.parquet")
     assert setup_cache.load(data, [data]) is None
@@ -176,6 +181,25 @@ def test_load_invalidates_on_file_content_change(tmp_path):
     assert setup_cache.load(data, files) is None
 
 
+def test_load_returns_none_on_malformed_cache_body(tmp_path):
+    """format_version/dims/fingerprint all check out, but the cache body
+    itself doesn't match SetupCache.from_json's expected shape (e.g. hand-
+    edited or written by a version that changed a nested key) — a clean
+    miss, not a crash."""
+    data = _touch_parquet(tmp_path / "shard.parquet")
+    files = [data]
+    setup_cache.save(data, files, SetupCache.empty(files))
+
+    path = setup_cache.sidecar_path(data)
+    raw = json.loads(path.read_text())
+    raw["vocab"] = {"pdg_map": {"11": 0}}  # missing required "mat_map" key
+    path.write_text(json.dumps(raw))
+
+    echoed = []
+    assert setup_cache.load(data, files, echo=echoed.append) is None
+    assert any("malformed" in m for m in echoed)
+
+
 def test_load_soft_warns_on_git_hash_mismatch_but_still_hits(tmp_path, capsys):
     data = _touch_parquet(tmp_path / "shard.parquet")
     files = [data]
@@ -352,3 +376,11 @@ def test_compute_event_index_from_files_single_file_unaffected(tmp_path):
 
     np.testing.assert_array_equal(unique_ids, [5, 7])
     np.testing.assert_array_equal(counts, [2, 1])
+
+
+def test_compute_event_index_from_files_empty_file_list():
+    unique_ids, counts = setup_cache.compute_event_index_from_files([])
+    assert unique_ids.size == 0
+    assert counts.size == 0
+    assert unique_ids.dtype == np.int64
+    assert counts.dtype == np.int64