nx_f32_llama_block_test.nx

nx_f32_llama_block_test.nx -- smoke for nx_f32_llama_block.nx. Identity-with-zero-weights test: All weight matrices are zero (mmap default). gamma_attn and gamma_ffn can be anything; with zero W_q/k/v the attention projections are 0; with zero W_o attn_proj=0. x_mid = x + 0 = x. FFN similarly produces 0 with zero W_*. out = x_mid + 0 = x. This exercises the full forward pass (RMSNorm + matmul + RoPE + attn-with-cache + SwiGLU + residual) and verifies the residual path preserves x bit-exact when weights are zero.