Project Medicare premiums and IRMAA past the last announced year

[MaxGhenis]

WIP. Stacked on #10085 (base premium-inputs-cms-uprating), which adds the Part B standard premium series this PR uprates by. It retargets to main once #10085 merges.

gov.hhs.medicare.part_b.base_premium stops at 2026 and has no uprating, so modeled Part B premiums, IRMAA, MSP benefits, SPM medical out-of-pocket spending and every downstream consumer hold $202.90 a month from 2027 on. The Part A premiums, the Part A deductible (still at 2025), the Part B and D IRMAA amounts and the IRMAA thresholds have the same freeze.

Plan

• Part B standard premium and Part B IRMAA amounts: uprate by the Trustees' projected standard premium (Uprate premium inputs by premium series, not CMS out-of-pocket spending #10085's calibration series), rounded to 10 cents.
• Part A full and reduced premiums and deductible: Trustees Table V.E1 series, with statutory rounding.
• IRMAA thresholds (Parts B and D): 42 U.S.C. 1395r(i)(5) CPI-U schedule.
• Part D IRMAA amounts: 42 U.S.C. 1395w-113(a)(7)(B) on the Trustees' base beneficiary premium, with the IRA's 2030 reset to 20%.
• Tests: Hypothesis properties, differential tests against Trustees Tables V.E1–V.E4 and CMS's published thresholds, YAML integration tests.
• Microsimulation impact for 2027 and 2034 under the hub heavy lock.
• Axiom parity issue.
• Independent review.

🤖 Generated with Claude Code

[MaxGhenis]

Coordination note from #10112, which adds the 2015–2023 IRMAA schedules and the 2015–2020 standard Part B premium.

• Same files. Add 2015-2023 Medicare IRMAA brackets and 2015-2020 Part B premiums #10112 edits the ten part_{b,d}/irmaa/*.yaml files and part_b/base_premium.yaml. It only adds dates before 2024 and does not touch 2024–2026. Whichever PR lands second has a mechanical conflict: it must move the new 2015–2023 entries under this PR's values: key.
• Projections unaffected. extend_medicare_irmaa_thresholds starts from the latest year in each values_list, so the earlier dates do not change your projections.
• 85% tier before 2019. The 85% bracket now has a .inf threshold for 2015–2018, because the Bipartisan Budget Act of 2018 added that tier for 2019. In 2018 the separate schedule's second bracket is also .inf.
• Axiom. The parity issue for the IRMAA schedule is Encode Medicare IRMAA: 42 U.S.C. 1395r(i) and 1395w-113(a)(7), with the 2015-2026 CMS schedules (PE #10112) TheAxiomFoundation/rulespec-us#1677 (queued). This PR's "Axiom parity issue" item can extend it rather than open a new one.

[MaxGhenis]

Heads-up from #10117, which adds the 2015–2020 Part A premiums and the 2015–2023 and 2026 Part A deductibles to the same three files.

• Conflicts: git merge-tree against this PR's head gives one conflict per Part A file (full_premium, reduced_premium, deductible). Each is "Add 2015-2020 Medicare Part A premiums and 2015-2023 Part A deductibles #10117 adds Federal Register reference lines after the statute reference, this PR adds nothing there". Keep the Add 2015-2020 Medicare Part A premiums and 2015-2023 Part A deductibles #10117 lines.
• Clean merges: Add 2015-2020 Medicare Part A premiums and 2015-2023 Part A deductibles #10117 adds the 42 U.S.C. § 1395i-2(d) / § 1395e(b)(1) references with exactly this PR's lines, plus the identical 2026 deductible value ($1,736) and its 2026 fact sheet reference, so those merge cleanly.
• Tests: Add 2015-2020 Medicare Part A premiums and 2015-2023 Part A deductibles #10117's new cases in base_part_a_premium.yaml sit before "unit test 5", not at the end, so this PR's appended 2027 cases don't collide.
• Possible differential test: calibration.gov.hhs.cms.medicare_part_a_deductible and medicare_part_a_premium_per_enrollee repeat the 2015–2026 amounts. Once both land, a test could check that they match the gov.hhs.medicare.part_a values for those years.
• Axiom: Encode Medicare Part A premiums and inpatient hospital deductible: 42 U.S.C. 1395i-2(d) and 1395e(b), with the 2015-2026 CMS amounts (PE #10117) TheAxiomFoundation/rulespec-us#1679 queues the Part A parity work (1395i-2(d), 1395e(b)). This PR can extend it.