ALLEN PARK, Mich. — Any way you slice it, the San Francisco 49ers are the NFL’s best offense. They rack up the most yards per play, the most points per drive, and rank first — by a wide margin — in DVOA, a down-by-down measure of efficiency. The team has four first-team All-Pros on the unit, as well as another second-teamer, and none of those are second-year quarterback Brock Purdy, who led the league in passer rating and QBR. He pilots an extraordinarily explosive passing game that finished tops in the NFL in gains of both 20-plus and 40-plus yards. It’s enough to wonder where you start game-…