Function fa_vec_min_tkv 

pub fn fa_vec_min_tkv() -> usize

Env-overridable crossover (MEMRA_FA_VEC_MIN, default FA_VEC_MIN_TKV). The 96 floor was measured on the qwen geometry (nkv=2); gemma4 SWA layers run nkv=8 = 4x the vec grid, which moves the crossover — sweep per model, adopt per the battery.