feat: add installment payment support to atomic swaps

[levi0005]

closes #407

• Add paid_amount: i128 and is_installment: bool fields to SwapRecord
• Implement submit_installment_payment(swap_id, payment_amount) buyer-only: transfers tokens to escrow, accumulates paid_amount, transitions swap to Accepted and emits swap_acpt event when paid_amount >= price
• Implement get_installment_status(swap_id) -> (paid, total, remaining)
• initiate_swap_with_schedule sets is_installment=true on the swap record
• Reject overpayment, zero payment, non-installment swaps, non-Pending swaps
• Add 7 tests: initial status, partial paid, fields stored, non-installment defaults, fully paid remaining=0, non-installment panics, overpayment panics

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