ar_withheld_income_tax ignores Arkansas's minus adjustments and exemption credit #9640

ar_withheld_income_tax computes Arkansas withholding as the bracket rate times the whole of reduced AGI:

rate = p.rates.main.rate.calc(reduced_agi)
return rate * reduced_agi

It never subtracts the minus adjustment. DFA's 2026 withholding formula does: at $26,400–$94,700, 3.7% minus $367.16. It also subtracts a $29 credit per withholding exemption. So modeled withholding is overstated in every year where the top rows carry a minus adjustment: by $367.16 plus the exemption credits for 2026 filers in the 3.7% row.

Once #9629 lands, ar_main_income_tax(reduced_agi, p.rates.main) in policyengine_us/variables/gov/states/ar/tax/income/ar_income_tax_helpers.py gives the full schedule, including the high-income table and bracket adjustment. Fix: use it, less the exemption credit, and add YAML tests against the DFA formula's worked example (PDF page 4).