Function sell_price

pub fn sell_price(eff: f64) -> i64

Item sale price (sell-currency units) from an item’s effectiveness eff: floor(eff^sell_price_exp · sell_price_coef). Sub-linear (exp < 1) so gold income grows ~linearly with item level and the geometric chest sink can bind (Phase 2). Pure + sim-tunable via tuning(); the call site is behaviors::items::item_price.