refactor(ml): add MLFilter.from_model(), fix validator initial_balance

- MLFilter.from_model() classmethod eliminates brittle __new__() private-attribute
  manipulation in backtester walk-forward model injection
- backtest_validator._check_invariants() now accepts cfg and uses cfg.initial_balance
  instead of a hardcoded 1000.0 for the negative-balance invariant check
- backtester.py walk-forward injection block simplified to use the new factory method

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

tests/test_ml_pipeline_fixes.py

@@ -100,3 +100,16 @@ def test_mlx_no_double_normalization():
 
     assert np.allclose(model._mean, 0.0), "normalize=False시 mean은 0이어야 한다"
     assert np.allclose(model._std, 1.0), "normalize=False시 std는 1이어야 한다"
+
+
+def test_ml_filter_from_model():
+    """MLFilter.from_model()로 LightGBM 모델을 주입할 수 있어야 한다."""
+    from src.ml_filter import MLFilter
+    from unittest.mock import MagicMock
+
+    mock_model = MagicMock()
+    mock_model.predict_proba.return_value = [[0.3, 0.7]]
+
+    mf = MLFilter.from_model(mock_model, threshold=0.55)
+    assert mf.is_model_loaded()
+    assert mf.active_backend == "LightGBM"