medfit: Infrastructure for Mediation Model Fitting and Extraction

Provides S7-based infrastructure for fitting mediation models, extracting path coefficients, and performing bootstrap inference. Designed as a foundation package for the mediation analysis ecosystem, supporting 'probmed', 'RMediation', and 'medrobust' packages. Implements unified interfaces for model fitting across different engines (currently generalized linear models, with future support for mixed models and Bayesian methods), standardized extraction of mediation paths from various model types, and robust bootstrap inference methods. Mediation inference methods are described in MacKinnon, Lockwood and Williams (2004) and Tofighi and MacKinnon (2011) .