Function gdn_update 

pub fn gdn_update(
    fp: FpUnit,
    row: &mut [f32],
    k: &[f32],
    decay: f32,
    beta: f32,
    v: f32,
    store: impl Fn(f32) -> f32,
) -> f32

One value row of a Gated-DeltaNet memory through one token’s rank-one update: coef = beta·(v − decay·(row·k)), each element decay·m + coef·k written back through store; returns coef.