feat(payments-engine): add buildTransactionParams for Stellar payments

[Creative-Titilayo]

Overview

This PR adds a buildTransactionParams function to @stellar-pay/payments-engine so payment flows can construct Stellar transaction parameters before signing and submission. It supports source account, destination, amount, asset, memo, and fee inputs, and returns built params ready for TransactionBuilder.

Related Issue

Closes #68

Changes

⚙️ Transaction Parameter Builder

• [ADD] packages/payments-engine/src/build-transaction-params.ts
• Added buildTransactionParams() to construct payment transaction parameters.
• Accepts source account, destination, amount, asset, memo, and fee.
• Returns built params including payment operation, fee, network passphrase, timeout, and encoded memo.
• Added encodePaymentMemo() with support for none, text, id, hash, and return memo types.
• Added buildTransactionFromParams() helper to assemble a transaction from built params.

📦 Public API Exports

• [MODIFY] packages/payments-engine/src/index.ts
• Exported PaymentAssetInput, PaymentMemoInput, PaymentMemoType, SourceAccountInput, BuildTransactionParamsInput, and BuiltTransactionParams.
• Re-exported buildTransactionParams, encodePaymentMemo, and buildTransactionFromParams.

Verification Results

pnpm --filter @stellar-pay/payments-engine build ✅ passed
pnpm --filter @stellar-pay/payments-engine typecheck ✅ passed

Acceptance Criteria	Status
Function accepts source account, destination, amount, asset, memo, and fee	✅
Returns built params ready for transaction building	✅
Includes memo encoding for different memo types	✅
Adds required interfaces to index.ts	✅
pnpm --filter @stellar-pay/payments-engine build passes with no TypeScript errors	✅

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