Add new Savings.fs module for ISA projections, fixed-term deposits and Premium Bonds

[Copilot]

Adds a new Savings module (src/Savings.fs) covering all three areas from the issue. All monetary values use the library's existing int64<Cent> precision model.

Changes Made

1. Regular savings / ISA projections

• project returns a month-by-month SavingsProjectionEntry array tracking opening/closing balance, contribution, interest earned, and running totals
• CompoundingFrequency: Daily, Monthly, Quarterly, Annually
• TaxWrapper: None, Isa (tax-free label, amounts unchanged), Lisa (25% government bonus applied to each contribution), PensionSalarySacrifice (configurable employer match rate)
• SavingsRate: Fixed (single rate for the full term) or Stepped (array of { FromMonth; AnnualRate } entries for variable/stepped-rate projections)

2. Fixed-term deposits

• projectDeposit returns a full per-month DepositProjectionEntry schedule and a DepositMaturityResult
• InterestPaymentTiming: AtMaturity (interest compounds monthly, paid as a lump sum at the end), Monthly / Annually (simple interest paid out periodically; principal stays fixed)
• grossToAer / aerToGross — AER ↔ gross rate conversion using the FCA-standard formula AER = (1 + r/n)^n − 1
• earlyWithdrawal — calculates gross interest to date, a configurable month-count penalty, and the net amount returned to the depositor

3. Premium Bonds expected return (stretch goal)

• premiumBondsExpectedReturn — expected annual prize income based on the NS&I prize fund rate
• comparePremiumBonds — returns a PremiumBondsComparison with: expected PB return, equivalent gross rate needed on a taxable account to match after tax, after-tax return on a taxable savings account at the same rate, and cash ISA return

Testing

• 21 new tests added in tests/SavingsTests.fs covering all sub-modules
• ✅ All 21 new tests pass
• ✅ No previously-passing tests broken (219 pre-existing failures in HTML generation code are unrelated and unchanged)

[simontreanor]

@copilot continue

[Copilot]

@copilot continue

Done. Commit 4e02b76 adds the full Savings module with:

1. Regular savings / ISA projections (RegularSavings)

• project — returns a month-by-month SavingsProjectionEntry array
• Compounding frequencies: Daily, Monthly, Quarterly, Annually
• Tax wrappers: None, Isa (tax-free label), Lisa (25 % govt bonus per contribution), PensionSalarySacrifice (employer match rate)
• Rate types: Fixed (single rate) or Stepped (schedule of { FromMonth; AnnualRate } entries)

2. Fixed-term deposits (FixedTermDeposit)

• projectDeposit — full per-month DepositProjectionEntry schedule + DepositMaturityResult
• Payment timings: AtMaturity (monthly compounding, lump sum at end), Monthly / Annually (simple interest paid out periodically, principal stays fixed)
• grossToAer / aerToGross — FCA-consistent AER ↔ gross rate conversion
• earlyWithdrawal — calculates gross interest, configurable month-count penalty, and net return

3. Premium Bonds (PremiumBonds)

• premiumBondsExpectedReturn — expected annual prize income
• comparePremiumBonds — PremiumBondsComparison with expected PB return, equivalent gross rate for a taxpayer, after-tax taxable savings return, and cash ISA return

21 new tests added in SavingsTests.fs; all pass. No existing tests broken.