Add the 2027 premium tax credit applicable percentages (Rev. Proc. 2026-26) #10442

The federal premium tax credit applicable percentages have no 2027 entry, so 2027 reuses the 2026 table (Rev. Proc. 2025-25). Rev. Proc. 2026-26 §3.01 sets the 2027 table:

§3.02 also sets the 2027 Required Contribution Percentage at 10.22%.

Where: policyengine_us/parameters/gov/aca/required_contribution_percentage/initial.yaml and final.yaml end at 2026-01-01. threshold.yaml needs no 2027 change: the bands match 2026.

Effect: in 2027 the model sets each Marketplace enrollee's required contribution too low, which overstates the premium tax credit. Programs built on the federal credit inherit the error. For example, the California Premium Subsidy is max(0, slcsp − aca_ptc − ca_pct × income), so each dollar of extra federal credit is a dollar less of state subsidy.

Worked example: the Covered California OE27 enroller deck's single filer at $31,920, exactly 200% FPL, with a $688 monthly benchmark.

• Official: a federal credit of $6,091.82 at 6.78%, and a state subsidy of $478.80.
• The model today: a federal credit of $6,149.28 at 6.60%, and a state subsidy of $421.34.

Done when:

• initial.yaml and final.yaml have 2027 values from Rev. Proc. 2026-26, with YAML cases at band edges.
• 2027 expectations that move are re-derived from the official table.
• A full-file 2027 impact run is in the PR.
• Amount Case 19 in policyengine_us/tests/policy/baseline/gov/states/ca/hbex/premium_subsidy/ca_premium_subsidy.yaml derives aca_ptc and expects the official figures above. That case arrives with Decide CA Premium Subsidy eligibility on the exact FPL ratio and adopt the Final 2027 design #10305, which supplies the federal credit until this lands.