Adds decode_step (KV cache + GPU sink-attention + MXFP4 experts) and
gemm::matmul_dense (cuBLAS without the m==1 GEMV shortcut). The
host-attention forward path is verified correct (top-1 " Paris"), but the
KV-cache DECODE path is still WRONG and non-deterministic: top-1 diverges
from the forward reference and varies run-to-run, generation is garbage.
matmul_dense did NOT fix it, so the m==1 GEMV atomicAdd theory was wrong
or incomplete. Root cause still open — debugging continues. Committing the
scaffolding so the WIP is captured; do not trust decode output yet.
Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>
Gahow Wang
afe7cc6645