New module: Analytics.fs (loan comparison, sensitivity analysis, currency support)

[Copilot]

Adds a new Analytics module providing higher-level decision-support functions built on top of the existing scheduling/calculation infrastructure, plus ISO 4217 currency support as a cross-cutting concern.

Currency support (Calculation.fs)

New Currency struct type and Currency module with correct ISO 4217 minor-unit rounding:

// 2 d.p.: GBP, USD, EUR  |  0 d.p.: JPY  |  3 d.p.: KWD, BHD, JOD, OMR, TND
Currency.fromMajorUnit Currency.KWD 1.234m  // → 1234L<Cent>
Currency.toMajorUnit   Currency.JPY 1234L<Cent>  // → 1234m  (no division)

Loan comparison (Analytics.LoanComparison)

LoanComparison.compare accepts (string * BasicParameters) array and returns a ComparisonResult with offers ranked four ways:

let result = LoanComparison.compare [|
    "Lender A", paramsAt 10m
    "Lender B", paramsAt 15m
|]
result.RankedByTotalCostOfCredit[0].Name  // "Lender A"
result.RankedByApr[0].Metrics.Apr         // Percent 10.xxxx

Ranked lists: RankedByTotalCostOfCredit, RankedByLevelPayment, RankedByApr, RankedByTotalAmountPayable. Ties broken by APR.

TotalCostOfCredit is defined as ScheduledPaymentTotal − Principal (fees + interest).

Sensitivity analysis (Analytics.SensitivityAnalysis)

SensitivityAnalysis.analyse takes a base BasicParameters and a Percent array of rate adjustments, returning a SensitivityScenario array — one row per adjustment — suitable for rendering as a table or chart:

SensitivityAnalysis.analyse baseParams [| Percent 0m; Percent 1m; Percent 2m; Percent 3m |]
// RateAdjustment | EffectiveRate      | TotalCostOfCredit | LevelPayment | Apr
// Percent 0m     | Annual (Percent 10m) | ...              | ...          | ...
// Percent 1m     | Annual (Percent 11m) | ...              | ...          | ...

Works with Annual and Daily rates; a Zero base rate with a non-zero adjustment produces an Annual rate.