Interest.fs: variable/stepped rate schedules and interest-only periods #35

Extend Interest.fs to support two common product structures not currently modelled:

1. Variable / stepped rate schedules

Repayment schedules where the interest rate changes at one or more future dates — e.g. tracker mortgages that follow Bank Rate, introductory-rate products, or planned rate switches at a fixed future date.

Proposed approach: Accept a RateSchedule (a list of (effectiveDate, rate) pairs) alongside the existing rate parameter. The amortisation engine applies each rate from its effective date forward.

2. Interest-only periods

Support for an initial interest-only phase followed by a capital-repayment phase — common in mortgages, bridging loans, and some consumer credit products.

Proposed approach: A repaymentType parameter on the schedule that allows InterestOnly | CapitalAndInterest | Mixed of interestOnlyMonths: int to model the transition between phases.

Both features should be composable (e.g. a tracker mortgage with a 2-year interest-only period).