Save and load a model
save_model.Rdsave_model() writes a glm_fit(), gam_fit() or elastic_net_fit()
model to an RDS file: the Rust fit as a versioned JSON artifact (spec,
estimates, covariance, fit statistics, fitted values, and provenance
with the package version and a hash of the training data; for a GAM
also the spline knots and constraints; for an elastic net one artifact
per lambda), with the formula terms and factor levels that
stats::predict() needs. load_model() reads it back; the estimates
round-trip exactly. A loaded model predicts and simulates but has no
training data, so robust_vcov() needs the original fit.