Add new module Arrears.fs for financial accounts modelling

[Copilot]

Adds src/Arrears.fs implementing FCA CONC 7 arrears, default interest, and forbearance modelling across three sections.

Changes Made

Section 1 – Arrears balance tracking

• ArrearsSummary struct: ArrearsBalance, ArrearsStartDate, DaysInArrears, MonthsInArrears, MissedPaymentCount, ArrearsInterest
• calculateArrearsSummary: derives the arrears position from an existing amortisation schedule; computes the running shortfall from missed/underpaid payments, days and months in arrears, and optional arrears interest at a configurable rate

Section 2 – Default / penal interest

• DefaultInterestRate DU: FixedRate of Rate | SpreadOverBase of BaseRate * Spread (spread adds annual percentage points to the base annual rate)
• DefaultInterestRate.toInterestRate: converts to a plain Interest.Rate
• applyDefaultInterest: injects the default rate as a promotional rate from a specified default date onward, returning a modified Parameters record

Section 3 – Repayment arrangement modelling

• ForbearanceTreatment DU: InterestFrozen | InterestReduced of Rate | InterestContinued
• ArrangementConfig record: arrangement start/end date, reduced payment amount, forbearance treatment, optional post-arrangement reschedule parameters
• ArrangementResult struct: ArrearsSummary, ArrangementSchedules, PostArrangementSchedules voption
• modelRepaymentArrangement: merges rescheduled reduced/suspended payments with forbearance promotional rates; optionally produces a return-to-normal schedule once the arrangement ends

Registration

• Arrears.fs added to src/FSharp.Finance.Personal.fsproj (after Refinancing.fs)
• ArrearsTests.fs added to tests/FSharp.Finance.Personal.Tests.fsproj

Testing

• ✅ Build succeeds with 0 warnings
• ✅ 14 new tests in tests/ArrearsTests.fs covering all three sections — all pass

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[Copilot]

@copilot continue

All three sections are complete and all 14 tests pass (commit 10566ed):

Section 1 – Arrears balance tracking (calculateArrearsSummary): computes the running arrears balance from missed/underpaid payments, days and months in arrears, missed-payment count, and arrears interest at a configurable rate.

Section 2 – Default/penal interest (applyDefaultInterest): supports FixedRate and SpreadOverBase default rates; the effective rate is injected as a promotional rate from the specified default date onward.

Section 3 – Repayment arrangement modelling (modelRepaymentArrangement): builds a merged arrangement schedule with configurable forbearance treatment (InterestFrozen / InterestReduced / InterestContinued) and optionally generates a return-to-normal schedule once the arrangement ends.