fix(graph): stop keeping every run's full state in the graph

- a graph reused across a backtest grid held each run's complete state for its whole life
- the state log is written from the run's own state; the ticker attribute it read is removed

tests/test_memory_log.py

@@ -932,7 +932,6 @@ class TestLegacyRemoval:
         }
         mock_graph = MagicMock()
         mock_graph.memory_log = TradingMemoryLog({"memory_log_path": str(tmp_path / "mem.md")})
-        mock_graph.log_states_dict = {}
         mock_graph.debug = False
         mock_graph.config = {"results_dir": str(tmp_path)}
         mock_graph.graph.invoke.return_value = fake_state