Tests scan SMEM safety (7/7 pass), delay parameter scaling from SM100,
and tile size comparison. Key insight: agent_scan.cuh _TempStorage is
a UNION — BlockLoad, BlockStore, BlockScan share SMEM. Peak = max(tile,
scan_scratch), NOT tile + scan_scratch.
8B structs at 99% SMEM utilization (48640/49152) are valid under union model.
Initial test had false SMEM overflow alarm (used sum model).