Function k64_n64_kernel 

pub fn k64_n64_kernel(gpu: &dyn GpuBackend) -> KernelHandle

Resolve the NARROW-N (N_TILE=64) deep-K twin. KernelHandle(0) when the kernel is absent or the presence kill switch ATLAS_NO_K64_N64 is set (=0 is NOT “off”). Callers must store the handle — kernel() is an init-time lookup, not a per-launch one.