fix: Prevent early loan impairment and due-date manipulation

Stop impairLoan and unimpairLoan from rewriting
sfNextPaymentDueDate when the amendment is active. Previously a
colluding broker could repeatedly impair and unimpair an overdue
loan to keep pushing the due date forward, permanently blocking
default eligibility and suppressing late-interest / late-fee
accrual.

include/xrpl/tx/transactors/lending/LendingHelpers.h

@@ -219,6 +219,10 @@ computeFullPaymentInterest(
     std::uint32_t startDate,
     TenthBips32 closeInterestRate);
 
+/// Returns true if the loan's next payment due date has passed.
+[[nodiscard]] bool
+isPaymentLate(ReadView const& view, SLE::const_ref loanSle);
+
 namespace detail {
 // These classes and functions should only be accessed by LendingHelper
 // functions and unit tests