Estimates both a vanilla two-way fixed effects (TWFE) model
and an extended TWFE (ETWFE) model, then selects between them using
Cochran's Q test for heterogeneity. When ETWFE wins, reports the
heterogeneity fraction (I-squared) and cohort-time estimates with
empirical Bayes shrinkage and Bonferroni multiplicity correction.
Methods build on Wooldridge (2025)