Function w4a16_v2_kernel 

pub fn w4a16_v2_kernel(gpu: &dyn GpuBackend) -> KernelHandle

2026-09-26: The w4a16_gemm_t_m128_v2 handle for the layer builders in dense_ffn_init.rs, qwen3_attention/init_prefill_kernels.rs and qwen3_ssm/init.rs.

KernelHandle(0) unless METRALE_W4A16_VARIANT is v2 or v3, with no lookup. For v2/v3 it panics when the target lacks the kernel.