Function select_layout 

pub fn select_layout(caller_is_tn: bool) -> GemmLayout

Select the layout to emit FROM THE PROBLEM: if the caller’s data is already TN (or we are free to choose the staging), prefer the tensor-core-native Tn for max throughput; if the data is fixed as Nn and a transpose would cost more than it saves, stay in Nn (where our kernel already beats cuBLAS). caller_is_tn = the operands are already available contraction-contiguous. This is the tradeoff knob, not a fixed choice.