Add calc_401kloan function for loan strategy optimization

[Copilot]

Implements financial calculator to determine optimal borrowing strategy between 401k loans and market loans by simulating account growth and payment schedules over the loan term.

Changes

• Core Function (R/calc-401kloan.R)

  • Compares three strategies: pure 401k loan, pure market loan with down payment, mixed approach
  • Monthly amortization with compound interest on remaining balances
  • Cash flow validation prevents recommending strategies that would deplete accounts
  • Configurable IRS 401k loan limits (default $50k or 50% of balance)
  • Returns optimal strategy with projected final balances

• Parameters

  • Loan amount, term (months), down payment percentage
  • Current 401k and cash balances
  • Annual yields for both accounts
  • APR rates for 401k and market loans

• Testing (tests/testthat/test-calc-401kloan.R)

  • Input validation, cash flow edge cases, various loan scenarios
  • Custom print method for readable output

• Documentation

  • Man pages with examples
  • Demo file with three realistic scenarios (car purchase, home down payment, short-term need)

Usage

result <- calc_401kloan(
  loan_amount = 30000,
  k401_balance = 100000,
  cash_balance = 20000,
  k401_yield = 0.07,
  cash_yield = 0.02,
  k401_loan_apr = 0.04,
  market_loan_apr = 0.08,
  n_months = 60
)

print(result)
# Strategy: 401k_only 
# Recommended 401k Loan: $30,000
# Expected Balance after 60 months: $160,890

The function selects the strategy that maximizes total final balance (401k + cash) after the loan period.

Warning

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Original prompt

I would likt to implement a function called calc_401kloan, with the following inputs: total loan amount, current balance in 401k retirement accounts, current cash balance, 401k average annual yield, cash yield amount, 401k loan apr, and market load apr. The goal is to calculate the best strategy for getting loans from 401k and market loans. Here is the strategy: the money loan from 401k will be paid back monthly at 401k loan apr. The reinvested money will grow monthly at 401k yield. An alternative way is to loan from market with downpayment (add this as input too but default is 0%). The existing cash is growing monthly, but paid interest will be lost. The number on months default is 60mo. Eventually returns the input parameters, how much i should loan from 401k, and what's the expected balance after 60mo. Also consider tax and early withdrawal penalty.

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