1. Return order: {threads, items} → {items, threads} matching CCCL scaling_result
2. Upper clamp: nominal*1 → nominal*2 (CCCL allows small types to double items)
3. Add threads SMEM cap: min(nominal, round_up(48KB/(ts*items), 32))
Verified against all 8 test vectors from CCCL catch2_test_util_arch.cu.
The old code was only safe because current bi100_* structs don't hit the
edge cases — but any future CCCL code copy would silently produce wrong
values.
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