End the senior and car loan deductions after 2028 at the source #10559

The senior deduction (26 U.S.C. 151(d)(5)(C)) and the car loan interest deduction (163(h)(4)) end after 2028, but additional_senior_deduction and auto_loan_interest_deduction keep computing in later years. The federal deduction lists drop them from 2029, so federal taxable income is right. Three states read the variables directly and get wrong 2029 values:

• South Carolina, through sc_obbba_addback.
• Montana, through mt_federal_obbba_subtraction.
• Arizona, which lists additional_senior_deduction from 2026.

Household probe at #10397's head f2d4122a, 2029: a single 67-year-old with $5,000 of car loan interest gets an SC add-back of $11,800, an MT subtraction of $11,000 and an AZ subtraction of $6,000.

#10397 fixes the same leak for the tips and overtime deductions with dated in_effect switches. The same pattern should fit here, with two differences to check:

• reforms/crfb/senior_deduction_extension.py extends the senior deduction past 2028, so it needs the new switch.
• Each state's law needs reading again for these two deductions. The state law section of End the tips and overtime deductions after 2028 at the source #10397 covers tips and overtime only.

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