Interest.fs: variable/stepped rate schedules and interest-only periods

[Copilot]

Two common product structures — tracker/stepped-rate schedules and interest-only phases — were not modelable in the library. This PR adds both as first-class, composable features.

Variable/Stepped Rate Schedules

Interest.BasicConfig gains a RateSchedule: RateSchedule field ((Date * Rate) array). Each entry overrides StandardRate from its effective date forward; promotional rates still take priority. Empty array = existing behaviour.

InterestConfig = {
    StandardRate = Rate.Annual(Percent 2.5m)  // applies before first step
    RateSchedule = [|
        Date(2025, 1, 1), Rate.Annual(Percent 3.5m)  // rate step at 12 months
        Date(2026, 1, 1), Rate.Annual(Percent 4.5m)  // further step at 24 months
    |]
    // ...
}

RateSchedule.effectiveRate picks the most recent entry with effectiveDate ≤ currentDate, sorting defensively against unsorted input.

Interest-Only Periods

AutoGenerateSchedule gains a RepaymentType field:

type RepaymentType =
    | CapitalAndInterest          // standard amortisation (default)
    | InterestOnly                // all payments = interest; final payment repays principal
    | Mixed of interestOnlyPeriods: int  // first N periods IO, remaining C&I via bisection

• CapitalAndInterest: existing bisection-based level payment, unchanged.
• InterestOnly: each scheduled payment = capped interest accrued; the final payment additionally clears the outstanding principal.
• Mixed n: first n payment items are interest-only; the remaining items are generated via the standard bisection solver starting from the full (unreduced) principal balance.

Both features compose naturally — a tracker mortgage with a 2-year interest-only period:

ScheduleConfig = AutoGenerateSchedule {
    UnitPeriodConfig = Monthly(1, 2024, 2, 1)
    ScheduleLength = PaymentCount 300
    RepaymentType = Mixed 24
}
InterestConfig = {
    StandardRate = Rate.Annual(Percent 2.5m)
    RateSchedule = [| Date(2026, 1, 1), Rate.Annual(Percent 3.5m) |]
    // ...
}

Backward Compatibility

All existing record constructions require the two new fields. Defaults for existing code: RateSchedule = [||] and RepaymentType = RepaymentType.CapitalAndInterest.