SEP-31 Cross-Border Payment Initiator

[Kingsman-99]

Description

Enterprise invoices sent to recipients in different countries require the SEP-31 direct payment standard for cross-border fiat settlement with a receiving anchor. The SDK has no SEP-31 integration, leaving integrators to implement the POST /send flow, required field resolution, and transaction status polling from scratch. A SEP-31 initiator should abstract the full initiator-side protocol flow.

Technical Context

Add src/sep/sep31Initiator.ts. Uses @stellar/stellar-sdk StellarToml.Resolver to fetch the receiving anchor's DIRECT_PAYMENT_SERVER URL. Calls /info to retrieve required fields, /send to initiate the payment, and polls /transaction for status. Integrates with src/token.ts for the SEP-10 JWT. Adds Sep31PaymentRecord and Sep31Status to src/types.ts. Emits sep31StatusChanged via src/events.ts.

Acceptance Criteria

• Sep31Initiator.initiate(asset, amount, receiverInfo, senderInfo, receiverAnchorDomain): Promise<Sep31PaymentRecord> completes the /send call and stores the returned transaction ID
• Sep31Initiator.getRequiredFields(anchorDomain, asset): Promise<Sep31FieldDefinitions> calls the anchor /info endpoint and returns the typed field schema
• Sep31Initiator.pollStatus(transactionId, anchorDomain): AsyncIterableIterator<Sep31Status> yields status updates until a terminal state (completed or error)
• SEP-10 JWT is automatically attached to all requests from src/token.ts
• Unit tests mock the anchor REST endpoints and assert the full initiate-to-complete flow
• All CI checks (npm test, npm run build, ESLint/TypeScript) pass and the branch has no merge conflicts

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