Add statement generation to Amortisation.fs

[Copilot]

Adds a month-by-month account statement generation feature to Amortisation.fs, as a prerequisite for regulatory disclosure documents (e.g. SECCI, CCA s.77A).

Changes Made

src/Amortisation.fs

• StatementLine record: 12 fields covering Date, OpeningBalance, Advance, InterestCharged, ChargesIncurred, Payment, PrincipalPortion, FeePortion, InterestPortion, ChargesPortion, ClosingBalance, and CumulativeInterestPaid
• Statement.generate: converts an AmortisationSchedule into a StatementLine array, carrying forward opening/closing balances and a running cumulative interest total
• Statement.toPlainText: renders the statement as a fixed-width right-aligned plain-text table (suitable for inclusion in a letter or email)
• Statement.toCsv: renders the statement as RFC 4180-compliant CSV, quoting any field that contains a comma, double-quote, or newline

tests/StatementTests.fs (new file)

23 tests covering:

• generate: line count, opening/closing balance chain, zero opening balance, zero final closing balance when fully repaid, monotonically non-decreasing cumulative interest, date alignment, advance amount, cumulative interest equals sum of interest portions; add-on interest variants
• toPlainText: header column names, separator row, line count, fixed-width table invariant, day-0 data row content
• toCsv: header prefix, header column count, line count, comma-containing amounts are quoted, day-0 CSV row content, RFC 4180-aware column-count invariant (correct "" escape handling)

tests/FSharp.Finance.Personal.Tests.fsproj

• Registered StatementTests.fs in the compile list

Testing

• ✅ Build passes for netstandard2.0 and netstandard2.1
• ✅ All 23 new statement tests pass
• ✅ No regressions introduced (219 pre-existing unrelated failures unchanged)

[simontreanor]

@copilot continue

[Copilot]

@copilot continue

Completed. The implementation adds StatementLine and Statement (with generate, toPlainText, toCsv) to Amortisation.fs, along with 23 tests in StatementTests.fs. Also addressed the code-review feedback:

• Added a toPlainText data-row content test verifying specific formatted amounts for a known schedule
• Added an RFC 4180-aware CSV column-count test with a correct state machine that handles the "" escape sequence inside quoted fields
• Fixed a misleading comment in the double-quote test

Commits: 2feaada, ca576a0