additional_tax_bracket reform: from 2026 its brackets 1-7 are not current law, and a finite bracket 8 threshold untaxes income above it #9832

Two problems with the additional_tax_bracket contrib reform. Neither is fixed by #9831, which fixes only the NaN while the added bracket is unset.

1. From 2026 the reform's brackets 1 to 7 are not current law

The reform taxes ordinary income entirely on its own schedule: regular_tax_before_credits, income_tax_main_rates and taxable_income_taxed_below_25_percent all read parameters(period).gov.contrib.additional_tax_bracket.bracket, policyengine_us/parameters/gov/contrib/additional_tax_bracket/bracket.yaml.

From 2026 that file departs from the baseline (gov.irs.income.bracket):

• Several of the contrib's 2026+ thresholds carry a # CBO forecast. comment.
• Its 2026 married-filing-separately thresholds are non-monotone: bracket 5's 541,550 is above bracket 6's 305,875. The income_tax_main_rates returns large negative values (−$81.7k on $8.2k taxable income) under TCJA-expiration parameters; NE/CO/OR conformity imports it into state tax #9084 clamp neutralizes that.
• So turning the reform on for 2026 or later changes brackets 1 to 7 as well as adding bracket 8.
• From 2018 to 2025 the contrib values match the baseline in every rate and threshold, except one: the 2022 single bracket 6 threshold is 539,000 in the contrib file and 539,900 in the baseline.

Possible fix. Have the reform take brackets 1 to 7 from gov.irs.income.bracket, and keep only bracket 7's threshold and bracket 8's rate in the contrib file. Alternatively, refresh the file's 2026+ values to current law.

• Either change moves the 2026 YAML case added in Give the additional_tax_bracket reform's unset bracket zero tax instead of NaN #9831: "Unset added bracket, default 2026 parameters, joint filer", $324,821 at the file's current schedule.
• It also moves Tax 28% rate and unrecaptured 1250 gain at regular rates below the 24% bracket top, and use Schedule D Tax Worksheet line 21 on Form 6251 line 27 #9792's case "Additional tax bracket reform's own amount taxed below 25 percent", whose 49,300 is the top of the file's 2026 15% bracket.

2. A finite bracket 8 threshold leaves income above it untaxed

tax_at_main_rates (now built on amounts_in_brackets) taxes nothing above the last bracket's threshold. The baseline's last threshold is always infinite, but gov.contrib.additional_tax_bracket.bracket.thresholds.8.* is user-editable. Setting it to a finite value silently untaxes all income above it.

Possible fix. Treat the last bracket's top as infinity in the walk, or validate that the last threshold is infinite.

Found while fixing #9831; evidence in the build report for that PR.