Improve imputation of auto loan interest eligible for new OBBBA deduction

[martinholmer]

In the discussion of merged PR #397, @donboyd5 mentioned this issue:

2. Auto loan interest (see Q7 item 7C, below): This seems substantially overestimated and unfortunately we don't have a target to bring end results into line. Q9 gives Claude's thoughts on adjustments that might be made. I don't think Claude got everything right, but a variant on Claude's new vehicles approach (excluding interest on used cars or previously bought new cars) probably makes sense, as does an adjustment to try to limit it to cars with final assembly in the U.S. I assume the cost of this would grow over time, if it remains in law, as a greater and greater share of cars with financing were purchased after 12/31/2024. But we still want to exclude interest expense for used cars.

While all these points are sensible, at the current time there is a shortage of reliable estimates of the new auto loan interest deduction, which makes doing anything more that what is done in PR #397 speculative. This issue will be reassessed after better estimates of the deduction become available from JCT, IRS, or private tax-analysis groups.