End-to-end micro-bench for the production decoder block at res=32 ch=64
with seqlens=[256, 192, 128, 64]. Three stages reported (convs-only /
attn-only / combined block) at fp16, vs the all-SDPA-padded baseline.
Used to track the cumulative effect of the mtlgemm flash-attention-v2
fwd + bwd work on the actual decoder hot path.
