New module: Mortgage.fs — offset, equity release, and shared ownership calculations

[Copilot]

Adds a Mortgage module implementing three specialist mortgage product structures not previously modelled in the library.

Offset Mortgages

• calculateOffsetMortgage: net interest-bearing balance (mortgage − linked savings), monthly payment/interest saving, estimated term reduction, and total interest comparison.

Equity Release / Lifetime Mortgages

• calculateEquityRelease: daily-compounding balance projection to a future date, voluntary partial repayments applied on their respective dates, and no-negative-equity guarantee (NNEG) modelling (balance capped at property value, with a trigger flag).

Shared Ownership

• calculateSharedOwnershipOutgoing: blended monthly outgoing = mortgage payment + rent on unowned share.
• calculateStaircasing: cost of an additional tranche, updated owned/unowned shares, reduced rent, and new mortgage payment after staircasing.
• calculateTotalCostComparison: total cost over a given horizon vs. equivalent outright-purchase mortgage.

Usage sketch

open FSharp.Finance.Personal.Mortgage

// Offset mortgage
let result = calculateOffsetMortgage {
    MortgageBalance       = 200_000_00L<Cent>
    LinkedSavingsBalance  =  50_000_00L<Cent>
    AnnualInterestRate    = Percent 3m
    RemainingTermMonths   = 240
    MonthlyPayment        = ValueNone
}
// result.NetInterestBearingBalance = 150_000_00L<Cent>
// result.EstimatedMonthsSaved      = ~55 months

// Equity release — 40 years at 7%, NNEG cap at £200k
let er = calculateEquityRelease {
    InitialAdvance    = 100_000_00L<Cent>
    StartDate         = Date(2000, 1, 1)
    AnnualInterestRate = Percent 7m
    ProjectionDate    = Date(2040, 1, 1)
    PartialRepayments = [||]
    PropertyValue     = ValueSome 200_000_00L<Cent>
}
// er.NnegTriggered    = true
// er.BalanceAfterNneg = 200_000_00L<Cent>

All monetary values use int64<Cent> consistent with the rest of the library; percentages use Percent.