Complete Michigan expanded retirement deduction phase-in for 2026

[jindal-git]

Closes #9073

Description

Under Michigan's "Lowering MI Costs Plan" (Public Act 4 of 2023 / MCL 206.30(10)), the phase-out of the state's three-tier retirement tax system reaches completion in tax year 2026:

1. Lifted upper birth-year cutoff for 2026 and later so all filers born after 1945 qualify for the expanded deduction (birth_year.yaml).
2. Added official 2026 inflation-adjusted maximum subtraction limits ($67,610 for single/separate/HOH/surviving spouse and $135,220 for married filing jointly) to amount.yaml.
3. Added YAML test cases covering 2026 eligibility and deduction amounts.
4. Added towncrier changelog fragment in changelog.d/.

Tests

• uv run policyengine-core test policyengine_us/tests/policy/baseline/gov/states/mi/tax/income/deductions/retirement/expanded/ -c policyengine_us -v (13 passed)
• Full Michigan baseline test suite (552 passed)
• make format ran cleanly