achieveGap 0.1.0

Initial release

• First public release of the achieveGap package.
• Implements hierarchical penalized spline models for estimating achievement gap trajectories in longitudinal educational data.
• Provides the main function gap_trajectory() for fitting joint mixed-effects spline models with REML smoothing selection.
• Includes simultaneous confidence bands based on posterior simulation.
• Supports visualization and summary methods for estimated gap trajectories.
• Includes simulation utilities for benchmarking model performance.