Apr.fs: add UnitedKingdom.representativeApr for UK CCA advertising

[Copilot]

UK lenders advertising credit products must quote a representative APR (the rate at or below which ≥51% of accepted applicants are offered) derived via FCA CONC App 1.2.6 / CCA 1974 actuarial methodology. No function existed to derive this from a nominal rate + fee + term.

Changes

src/Apr.fs

• Adds UnitedKingdom.representativeApr to the existing UnitedKingdom module:
  • Computes equal monthly payments via the annuity formula on principal + fee at nominalAnnualRate / 12
  • Rounds to the nearest cent via Cent.fromDecimal
  • Solves the representative APR on the net advance (principal) using the existing calculateAprEuUk (actual-days UK actuarial method)
  • Returns Solution.Impossible for zero/negative term or zero principal

// £1,000 loan, 20% nominal p.a., 12 months, £50 arrangement fee
// Gross financed: £1,050 → monthly repayment: £97.27
// Representative APR solved on £1,000 net advance → ≈ 34.1%
let apr =
    Apr.UnitedKingdom.representativeApr
        (Date(2025, 1, 1))
        (Percent 20m)
        100_000L<Cent>   // £1,000 principal
        12
        5_000L<Cent>     // £50 fee

tests/AprUnitedKingdomTests.fs

New RepresentativeApr module with five tests:

Scenario	Principal	Nominal	Term	Fee	Expected APR
No-fee baseline	£1,000	20%	12 mo	£0	≈ 22.03%
Fee raises APR (invariant)	£1,000	20%	12 mo	£50	> no-fee APR
FCA CCA worked example	£1,000	20%	12 mo	£50	≈ 34.08%
Zero term	—	—	0	—	Impossible
Zero principal	—	—	—	—	Impossible