Only 3 changes from upstream_ref/sgemm_cuda/6_kernel_vectorize.cuh:
1. float → __half for A/B/C data and shared memory
2. float4 vectorized load → 4 scalar half loads (float4 needs 16-byte align)
3. threadResults accumulator stays float (FP32 accumulation)
Everything else identical: same shared mem layout, same indexing,
same A-transpose-while-loading, same thread tile computation.
No WARPSIZE. No cooperative_groups. No cuda::barrier.
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