[codex] Remove 2035 sunset from non-refundable SS credit

[MaxGhenis]

What changed

• remove the 2035-12-31 sunset from the gov.irs.credits.non_refundable rewrite in non_refundable_ss_credit_reform
• add a late-year policy regression showing the Social Security credit still flows through income_tax_non_refundable_credits in 2073
• add the required changelog fragment

Why

The reform-specific parameter rewrite was stopping in 2035. After that point, ss_credit could still be computed, but it no longer fed into the non-refundable credit stack. In downstream CRFB long-horizon scoring, that made late-year reforms collapse together incorrectly.

The CRFB deliverable is a 75-year horizon through 2100, and I do not see evidence in the CRFB thread that this custom SS credit reform was intended to sunset in 2035.

Validation

• uv run python -m policyengine_core.scripts.policyengine_command test policyengine_us/tests/policy/contrib/crfb/non_refundable_ss_credit.yaml -c policyengine_us
• result: 11 passed
• direct single-household spot checks for 2026 and 2073 confirming income_tax_non_refundable_credits remains active in the late-year case