Persist harness candidate set snapshots

tests/test_core_flow.py

@@ -6297,6 +6297,19 @@ class CoreFlowTests(unittest.TestCase):
             self.assertTrue(proposal_path.exists())
             proposal = json.loads(proposal_path.read_text(encoding="utf-8"))
             self.assertTrue(proposal["should_stop"])
+            snapshot_path = (
+                store.study_root(study.study_id)
+                / "harness"
+                / "candidate-set-0005.json"
+            )
+            self.assertTrue(snapshot_path.exists())
+            snapshot = json.loads(snapshot_path.read_text(encoding="utf-8"))
+            self.assertEqual(snapshot["schema_version"], 1)
+            self.assertEqual(snapshot["iteration"], 5)
+            self.assertIn("candidate_set_hash", snapshot)
+            self.assertIn("candidate_set", snapshot)
+            self.assertIn("harness_stop", snapshot["decisions"])
+            self.assertIn("stop_authority", snapshot["decisions"])
             state = store.load_state(study.study_id)
             self.assertEqual(state.tuning_stop_reason, "harness_stop")
             self.assertEqual(