Trustees 2025 scenario: confirm the horizon and re-derive the wage base and wage-indexed Social Security amounts from the Trustees NAWI #10452

Found in @PavelMakarchuk's review of #10227 (G9 and "Out of scope"). This is pre-existing; the prior head had the same split.

Wage base and wage-indexed amounts

apply_trustees_2025_economic_assumptions (policyengine_us/reforms/ssa/trustees_2025.py) re-extends the payroll cap from the Trustees NAWI path with extend_social_security_payroll_cap. It does not rerun extend_social_security_wage_indexed_parameters. In the Trustees parameter tree:

• gov.ssa.social_security.wage_base therefore keeps the CBO-derived cap. It differs from gov.irs.payroll.social_security.cap in all 65 years from 2036 to 2100: in 2036, 260,400 against 261,900, and in 2100, 1,995,300 against 2,454,900.
• The wage-indexed amounts stay on the CBO path too. The 2040 PIA bend point 1 stays 2,059, where the Trustees NAWI gives 2,107.

Fix: call extend_social_security_wage_indexed_parameters after the cap extension, and test that wage_base equals the cap.

Horizon (for the owner of #8220)

#10227 moved CBO's horizon under the scenario. CBO-based NAWI now runs through 2034 and the cap through 2036. The scenario's NAWI start_year is 2034 (trustees_2025.py:19 and :121) and its cap last_projected_year is 2035 (:200). So it now replaces CBO's 2034 NAWI (98,514.73) and CBO's 2036 cap.

The pair is consistent with itself. @MaxGhenis, is that the intent? The alternative is to follow CBO's longer window: start_year 2035 and last_projected_year=2036. The SOI projection at :151 already uses 2036.

Plan

• Re-derive the wage base and wage-indexed amounts from the Trustees NAWI (follow-up PR).
• Confirm the horizon or move it. The follow-up PR leaves it as is.