gpss 1.0.0

• Initial CRAN release.
• Core GP functions: gp_train(), gp_predict(), and formula interface gpss() with predict() method.
• Regression discontinuity design via gp_rdd() and gp_rdd_plot().
• Interrupted time-series design via gp_its() with optional placebo checks.
• Multiple kernel types: Gaussian, Gaussian-linear, Gaussian-quadratic, Gaussian-periodic-linear, and Gaussian-periodic-quadratic.
• Automatic bandwidth selection via kernel variance maximization.
• Optional noise variance optimization via marginal likelihood.
• Support for mixed continuous/categorical covariates.
• Prior mean support for incorporating external predictions.
• Companion paper: Cho, Kim, and Hazlett (2026), “Inference at the data’s edge: Gaussian processes for modeling and inference under model-dependency, poor overlap, and extrapolation,” Political Analysis, doi:10.1017/pan.2026.10032.