More hand computed test cases at the cliff edges

The test suite is hand computed cases pinned to published marginal rates. The most valuable additions sit exactly at the discontinuities. One dollar either side of each IRMAA tier, the ends of the Social Security taxability worksheet where the 85 percent cap engages and disengages, the senior deduction phase out endpoints, and the capital gains stacking boundary where qualified dividends cross from the 0 percent to the 15 percent rate. Each case should carry a comment showing the arithmetic by hand, the way the existing cases do, so a reviewer can check it without running anything.