Roblox monetization: safe ProcessReceipt + Gamepasses

Real Robux is unforgiving. A subtle bug here means players get charged and receive nothing (or get infinite free stuff). These are the patterns that keep it correct.

Developer Products: MarketplaceService.ProcessReceipt

The callback runs when a Robux DevProduct purchase completes. It MUST be idempotent (Roblox re-delivers until you return PurchaseGranted) and atomic (grant + dedupe recorded in the SAME durable save).