Can team strength change mid-season? Design for a time-varying model

The shipping Dixon-Coles fit (scripts/fit_dixon_coles.py) gives every team one attack parameter α_t and one defense parameter β_t for the entire fitting window. Time-decay is applied on the match-likelihood weights (5-year half-life on each match's contribution), but the parameters themselves are stationary — a fit that absorbs both the 2018 Argentina and the 2022 Argentina produces a single compromise. The question this note scopes: would per-team parameters that **vary through ti…