maxtwin 0.1.0

• Initial release.
• Added maxtwin() for deterministic train/test splitting with distribution preservation and train/test separation.
• Added initialmaxtwin() for the Step 1 split without the iterative separation improvement.
• Added input validation, examples, and test coverage for deterministic output, split sizes, paired data, and high-dimensional inputs.
• Added Helmert coding for factor predictors and factor responses.
• Added predictor-only splitting when Y is omitted.