feat(payments-engine): add closePaymentChannel for payment channels

[Creative-Titilayo]

Overview

This PR adds a closePaymentChannel function to @stellar-pay/payments-engine to finalize an existing payment channel, distribute escrowed funds to channel parties, collect required multi-party signoffs, and return the final channel state with the settlement transaction hash.

Related Issue

Closes #78

Changes

⚙️ Payment Channel Close Flow

• [ADD] packages/payments-engine/src/payment-channel.ts
• Added closePaymentChannel(channel, server) to finalize and close payment channels.
• Added buildChannelCloseTransaction() to construct settlement payment operations from escrow.
• Validates channel state, distribution targets, and signer thresholds before submission.
• Collects multi-party keypair signoffs and submits the close transaction via Horizon.
• Returns ChannelCloseResult with final closed status and transaction hash.

📦 Public API Exports

• [MODIFY] packages/payments-engine/src/index.ts
• Exported PaymentChannel, PaymentChannelSigner, PaymentChannelDistribution, PaymentChannelAsset, PaymentChannelStatus, ChannelCloseResult, and Horizon.
• Re-exported closePaymentChannel and buildChannelCloseTransaction.

Verification Results

pnpm --filter @stellar-pay/payments-engine build ✅ passed
runtime verification ✅ buildChannelCloseTransaction creates settlement ops and signatures
runtime verification ✅ closePaymentChannel returns closed status + transaction hash
runtime verification ✅ insufficient signoffs rejected with clear error

Acceptance Criteria	Status
closePaymentChannel(channel, server) implemented	✅
Finalizes channel and distributes funds	✅
Handles multi-party signoffs	✅
Returns final channel state and transaction hash	✅
ChannelCloseResult interface added	✅
pnpm --filter @stellar-pay/payments-engine build passes	✅

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