Add touchdown-luck regression as a new environment multiplier

Red zone touches and touchdowns, pulled from raw play-by-play, regressed
toward what a player's position converts on average (same starter-caliber
cohort pos_target already uses). Surfaces as m_td_luck alongside the other
bounded environment factors, tunable via model_settings(td_luck_weight=...),
and shown in player_report / explain()'s summary as "touchdown regression".

Below 8 red zone touches a rate is noise, not signal, and sits neutral.