Staggered DiD Tools: Event Studies and ATT Aggregation

Provides tools for difference-in-differences (DiD) estimation and visualization with staggered adoption. Includes run_es() for event-study curves (dynamic effects by relative time) and calc_att() for aggregated ATT estimation (overall, by cohort, by calendar time). Supports multiple modern estimators: Callaway-Sant'Anna (2021), Sun-Abraham (2021), Borusyak-Jaravel-Spiess (2024), Wooldridge TWM, and Deb et al. FLEX.