Refinancing: add remortgage cost comparison and debt consolidation NPV

[Copilot]

Extends Refinancing.fs with two borrower/adviser comparison calculations that were missing from the module.

compareRemortgage

Compares the total cost of staying on a current mortgage product vs. switching to a new one.

New types: RemortgageComparisonParameters, RemortgageComparisonResult

let rcp: RemortgageComparisonParameters = {
    EarlyRepaymentCharge = Amount.Percentage(Percent 2m, Restriction.NoLimit)
    ArrangementFee = 999_00L<Cent>
    LegalCosts = 300_00L<Cent>
    NewProductParameters = newProductParams  // principal = outstanding balance
}

let result = compareRemortgage currentParams actualPayments rcp
// result.NetBenefit           – positive = switching saves money
// result.BreakEvenPeriodCount – ValueNone if savings never recover upfront costs within remaining term

• Outstanding balance sourced from getQuote (settlement quote on evaluation day)
• ERC computed as an Amount against the outstanding capital balance
• Break-even = ⌈upfrontCosts / perPeriodSaving⌉; ValueNone if new product isn't cheaper per period or break-even exceeds remaining term

compareDebtConsolidation

Compares the NPV of repaying multiple existing debts individually against replacing them with a single consolidated facility, discounting future cash flows at a caller-supplied rate.

New types: DebtConsolidationParameters, DebtConsolidationResult

let dcp: DebtConsolidationParameters = {
    DiscountRate = Interest.Rate.Annual(Percent 5m)
    ConsolidatedFacilityParameters = consolidatedParams
    ConsolidationFees = 500_00L<Cent>
}

let result = compareDebtConsolidation existingDebts dcp
// result.IndividualPaymentsNpv    – PV of paying each debt separately
// result.ConsolidatedPaymentsNpv  – PV of consolidated payments + upfront fees
// result.NetConsolidationSaving   – positive = consolidation is more economical

• Discount factors computed once per unique day-count (memoised) to avoid redundant Math.Pow calls
• Supports Interest.Rate.Zero, Annual, and Daily discount rates
• Upfront consolidation fees added at discount factor = 1 (paid at reference date)
• Both strategies filter to payments strictly after each debt's evaluation day for a consistent present-value comparison