Survivor / widow(er)'s-penalty tax transition projection

Projects the "widow's penalty": when the first spouse dies, the survivor files single instead of jointly — brackets compress, the §86 Social Security and IRMAA thresholds roughly halve, and one Social Security benefit stops. Prices the household both ways (MFJ while both alive vs the survivor filing single) by composing the tax, sstax, and irmaa engines, and reports each scenario's AGI / MAGI / taxable SS / income tax / IRMAA, the annual widow's penalty, the income lost, and the marginal-rate jump. With conversion_amount, it also values accelerating a Roth conversion into the cheaper still-MFJ years. Deterministic; not tax advice.