Function fa_apply 

pub fn fa_apply(
    graph: &CudaGraph,
    plan: &mut [FaMain],
    t_kv: usize,
    split_keys: impl Fn(usize, usize) -> usize,
) -> Result<(), Box<dyn Error>>

Retune every fa main (and paired combine) in the instantiated exec to the live t_kv: vec mains get the EAGER split count ns = ceil(t_kv/split_keys(t_kv, nkv)); scalar mains shrink grid.y to their in-kernel ns_eff. No-op when the counts haven’t stepped. split_keys is the caller’s ladder (fa_split_keys) so graph and eager stay in lockstep.