Add Apr.CalculationMethod.Disabled for non-APR products

[Copilot]

Zero-interest products (salary advances, BNPL) have no meaningful APR, but the current CalculationMethod DU had no way to express this — requiring a misleading placeholder.

Changes

• src/Apr.fs

  • Added | Disabled case to CalculationMethod with doc comment
  • Html member returns "n/a" for Disabled
  • calculate returns Solution.Impossible (rather than throwing or returning zero)
  • toPercent made exhaustive — UnitedStatesRule and Disabled both explicitly yield precision 0

• tests/AprDisabledTests.fs (new): 4 tests covering calculate, toPercent, and Html for Disabled

Usage

let interestConfig = {
    Method = Interest.Method.None
    StandardRate = Interest.Rate.Zero
    Cap = Interest.Cap.zero
    Rounding = RoundDown
    AprMethod = Apr.CalculationMethod.Disabled  // clearly opt out of APR
}

// calculate returns Solution.Impossible; toPercent returns Percent 0m
Apr.calculate Apr.CalculationMethod.Disabled principal startDate transfers
// => Solution.Impossible