Add payment holidays and partial overpayments to Refinancing.fs

[Copilot]

Extends Refinancing.fs with two new real-world lending scenarios.

Changes Made

Payment holidays / deferrals (paymentHoliday)

• DeferredInterestHandling DU: CapitaliseAsNewPrincipal | SpreadOverRemainingTerm
• PaymentHolidayParameters record: HolidayStartDay, HolidayEndDay, InterestHandling, SettlementDay
• paymentHoliday function: zeroes scheduled payments during the holiday window and recomputes the post-holiday schedule using the chosen interest strategy — capitalising accrued holiday interest into the outstanding balance, or spreading the equivalent payment increase evenly over the remaining term

Partial overpayments (applyOverpayment)

• OverpaymentHandling DU: ReduceTerm | ReducePayment
• OverpaymentParameters record: OverpaymentHandling, SettlementDay
• applyOverpayment function: recomputes the remaining schedule after an overpayment to either shorten the term (same payment amount) or lower the regular payment amount (same number of remaining payments)

Both functions return {| OldSchedules; NewSchedules |} consistent with the existing reschedule and rollOver functions, so statement generation and APR recalculation work correctly on the modified schedule.

Tests

New tests/RefinancingTests.fs with 10 passing tests covering all four scenarios and key edge cases (holiday window with no scheduled payments, over-clearing overpayment). Registered in FSharp.Finance.Personal.Tests.fsproj.

Testing

• ✅ Library builds successfully (dotnet build src/FSharp.Finance.Personal.fsproj)
• ✅ All 10 new RefinancingTests pass
• ✅ 219 pre-existing test failures are unchanged (unrelated HTML-generation issue)

[simontreanor]

@copilot continue

[Copilot]

@copilot continue

Continued and completed. The root cause was that ReducePayment's level-payment calculation was anchoring the first generated payment on the evaluation date (day 0) instead of the first remaining scheduled payment date (one period later), so the interest timing was off and the final balance never reached zero. Fixed in commit 16610a9 by restoring firstRemainingDate as the unit-period anchor while keeping StartDate = EvaluationDate so pre-payment interest is still included.

All 10 new RefinancingTests now pass; the 219 pre-existing failures are unchanged.