varycoef 0.4.0

Improvements

• Added early validation for model, selection, and prediction inputs, with clearer error messages.
• Moved selection-specific packages to optional dependencies so core model fitting requires fewer packages.
• Improved covariance solve calculations and corrected covariance handling in predictive and tapered computations.
• Expanded regression tests for selection validation, formula and matrix fits, predictions, and optional dependencies.
• Added continuous integration checks for R release and development versions.

Some development improvements were implemented with assistance from GitHub Copilot. All changes were reviewed and validated by the package maintainer.