Roth vs traditional contribution comparator

Compares a Roth vs a traditional (pre-tax) retirement contribution on an apples-to-apples, after-tax basis. The same annual dollars go into either account; the traditional path also invests its up-front tax saving in a taxable side account (matching the out-of-pocket cost), whose gains are taxed at the end. Roth withdrawals are tax-free; traditional withdrawals are taxed at the retirement marginal rate. Returns each path's after-tax future value, the Roth advantage, the recommendation, and the breakeven retirement rate. Deterministic; not advice.