Fix get_irs_cpi 12-month CPI window - #9198
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… documents (and IRS practice uses) the 12 months ending August Closes PolicyEngine#9105
…ow; format The Sep-Aug window correction shifts gov.irs.uprating-projected values by a few dollars, so: RI 2027 SS-modification boundary cases move to the corrected projected limits (single 112,750, joint 140,950 — inputs shifted, intent preserved: at-limit stays ineligible, one dollar below stays eligible); RI high-earner, RI integration, MN HF4890, and CA AB2591 projected-value pins updated to the corrected-window outputs; ruff format on the new window test. Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>
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Issue #9105 reports that get_irs_cpi documents and intends to follow IRS practice by averaging chained CPI-U over the 12 months ending in August, but the implementation starts 12 months before the August instant and iterates 12 values. That selects August of the prior year through July of the target year, excluding the target year's August value.
This changes the loop start to 11 months before the August instant so the averaged window is September of the prior year through August of the target year, inclusive.
A regression test now computes the expected average from explicit September-August monthly C-CPI-U values and verifies get_irs_cpi matches that window, while demonstrating the result differs from the old August-July window.
Modified files:
ruff check policyengine_us/parameters/uprating_extensions.py policyengine_us/tests/policy/baseline/parameters/test_uprating_extensions.pyreports no new findings on the changed files.