Hydrological Evaluation Metrics and Goodness-of-Fit Summaries // Computes scalar performance metrics and goodness-of-fit summaries for comparing simulated and observed hydrological or regression values. Provides error metrics, percentage bias, Nash-Sutcliffe efficiency, Kling-Gupta efficiency variants, correlation measures, agreement indices, and comparison tables via gof() and gof_compare(). Metric definitions are registry- governed with shared validation, provenance-aware wrappers, and explicit handling of undefined or degenerate cases. Methods include Nash and Sutcliffe (1970) doi:10.1016/0022-1694(70)90255-6, Gupta et al. (2009) doi:10.1016/j.jhydrol.2009.08.003, Kling et al. (2012) doi:10.1016/j.jhydrol.2012.01.011, and Willmott et al. (2012) doi:10.1002/joc.2419.