Manajemen Risiko Portofolio Saham IDX BUMN20: Implementasi Model EWMA, GARCH dan Hybird EWMA-GARCH dalam Pengukuran dan Prediksi Value at Risk (VaR) = Risk Management of IDX BUMN20 Stock Portfolio: Implementation of EWMA, GARCH, and Hybird EWMA-GARCH Models in Value at Risk (VaR) Measurement and Prediction