IQCC - Improved Quality Control Charts

Implements improved statistical process control charts with exact, Cornish-Fisher corrected, standardized, and simulation-based probability limits. Methods include univariate attribute and variable charts, double-sampling charts for rare nonconformities, Hotelling T-squared charts, and generalized variance charts for multivariate process variability. Numerical functions for limits, false-alarm risk, average run length, and average sample size are separated from plotting interfaces to support reproducible statistical validation.