Universal Turning Point and Inflection Point Tests // Performs turning point and inflection point tests for U-shaped and inverse U-shaped relationships in regression models. Implements the Sasabuchi (1980) test as extended by Lind and Mehlum (2010) with support for quadratic, cubic, log-quadratic, and inverse functional forms. Features include delta-method standard errors, Fieller confidence intervals, Simonsohn (2018) two- lines test, and parametric bootstrap. Designed for post- estimation analysis of linear models, panel models, and quantile regression. References: Lind and Mehlum (2010) doi:10.1111/j.1468-0084.2009.00569.x; Sasabuchi (1980); Fieller (1954) doi:10.1111/j.2517-6161.1954.tb00159.x.