FluentCart payment gateways

Implement a gateway as a provider adapter around FluentCart's order, transaction, and subscription state machine. A successful browser callback is not proof of settlement.

Read gateway-contract.md before creating the PHP class, checkout JavaScript, webhook, or subscription branch.

Implement the required contract

Extend AbstractPaymentGateway and supply BaseGatewaySettings. Implement:

• meta()
• fields()
• makePaymentFromPaymentInstance()
• handleIPN()
• getOrderInfo()