ConstReadonlyCUSTOM_PROTOCOL_FEE: 4Bit 2: Custom protocol fee enabled — overrides default 100 bps (1 = enabled)
ReadonlyNET_AMOUNT: 2Bit 1: Net amount mode — recipient receives exactly payment_amount, fees added on top (1 = net)
ReadonlyREFERRAL: 1Bit 0: Referral program enabled (1 = enabled)
Gateway feature flag bit positions. Bit-vector stored in PaymentGateway.feature_flags (u8).