Puerto Rico refundable CTC subtracts SSI; Schedule 8812 line 24 is EITC plus excess Social Security/RRTA tax #10150

Found during the independent review of #10141. This was already on main; #10141 doesn't change it.

policyengine_us/variables/gov/territories/pr/tax/income/credits/refundable_ctc/pr_refundable_ctc.py (around lines 20-22) computes ssi = max_(0, add(tax_unit, period, ["uncapped_ssi"])). It treats Schedule 8812 line 24 as the EITC plus Supplemental Security Income.

On the official Schedule 8812 (https://www.irs.gov/pub/irs-pdf/f1040s8.pdf#page=2), line 24 adds the EITC and Schedule 3 line 11, which is excess Social Security and tier 1 RRTA tax withheld. SSI is not part of it.

Fix: read the current Schedule 8812 instructions for bona fide Puerto Rico residents in each year the variable covers. Replace SSI with the line's actual inputs, and add YAML cases built from the form's line arithmetic.