New module: BuyNowPayLater.fs — zero-interest instalments and deferred payment

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Adds a BuyNowPayLater module to model 0%-interest instalment ("pay in 3 / pay in 12") and deferred payment ("pay later") products, along with the prerequisite Apr.CalculationMethod.Disabled case for products where a conventional APR is not applicable.

Apr.CalculationMethod.Disabled (Apr.fs)

• New DU case; calculate returns Solution.Impossible, toPercent returns Percent 0m explicitly
• HTML label: "disabled"

BuyNowPayLater module (src/BuyNowPayLater.fs)

Types

• FundingModel — RetailerFunded | LenderFunded (who absorbs the 0% cost)
• InstalmentConfig — equal-payment instalment product; carries PenalInterestRate for use when a missed payment is detected
• DeferredConfig — deferred payment product; AccrueInterestDuringDeferral flag distinguishes true 0%-deferred from deferred-interest variants

Factory functions — all set AprMethod = Disabled and StandardRate = Zero:

// "pay in 3" — equal monthly payments, no interest
let config: InstalmentConfig = {
    InstalmentCount = 3
    UnitPeriodConfig = Monthly(1, 2024, 4, 1)
    PenalInterestRate = Interest.Rate.Annual(Percent 39.99m)
    FundingModel = FundingModel.RetailerFunded
}
let p = instalmentParameters startDate principal config
let schedule = calculateBasicSchedule p.Basic  // zero interest, zero APR

// "pay later in 90 days" with deferred interest
let config: DeferredConfig = {
    DeferralPeriodDays = 90<DurationDay>
    AccrueInterestDuringDeferral = true   // interest waived via promotional zero-rate if paid on time
    DeferredRate = Interest.Rate.Annual(Percent 34.9m)
    PostDeferralRate = Interest.Rate.Annual(Percent 34.9m)
    FundingModel = FundingModel.LenderFunded
}
let p = deferredParameters startDate principal config
// advanced params include promotional Rate.Zero covering the full deferral period

For deferred-interest products, deferredAdvancedParameters injects a PromotionalRate of Zero spanning the entire deferral window (including the payment day), so on-time amortisation shows zero net interest. The PostDeferralRate / PenalInterestRate are stored in config for callers to apply via PromotionalRates after a missed payment is detected.