Count the pre-2018 miscellaneous deduction once in Mississippi and Missouri itemized deductions

[MaxGhenis]

WIP (draft). Pending: targeted YAML runs (queued under the hub heavy lock), main-vs-branch households, an independent review and the Axiom parity item.

What changes

itemized_deductions_less_salt and itemized_taxable_income_deductions already include misc_deduction, the federal miscellaneous deduction above the 2% floor, which is nonzero only before 2018. Two states counted it a second time:

• Mississippi: ms_itemized_deductions_unit added misc_deduction on top of itemized_deductions_less_salt.
• Missouri: gov.states.mo.tax.income.deductions.itemized listed misc_deduction beside itemized_taxable_income_deductions.

Both forms take it once:

• 2017 Mississippi instructions (PDF p.15): miscellaneous expenses are deductible "only to the extent that such expenses exceed the 2% federal adjusted gross income limit", on Form 80-108 lines 7a–7d, which is the same amount as federal Schedule A line 27. https://taxsim.nber.org/historical_state_tax_forms/MS/2017/80100178.pdf#page=15
• 2017 Form MO-A Part 2 (PDF p.22): line 1 "Total federal itemized deductions from Federal Form 1040, Line 40", then social security, railroad retirement, Medicare and self-employment taxes on lines 2–7. There is no separate miscellaneous line. https://dor.mo.gov/forms/MO-1040%20Instructions_2017.pdf#page=22

From 2018 misc_deduction is zero (26 U.S.C. 67(g)), so only years before 2018 change.

Test edits

Two Missouri cases (2021) entered misc_deduction beside the federal total and expected the sum. Missouri no longer reads misc_deduction, so each case's two inputs are folded into itemized_taxable_income_deductions. Expected values are unchanged.

Coordination

#10070 edits the same adds list in ms_itemized_deductions_unit.py and creates the same test file. Whichever lands second keeps both changes.

Invariants

Mississippi and Missouri itemized deductions each include the federal miscellaneous deduction exactly once. From 2018 both are unchanged, because misc_deduction is zero.

Microsimulation

Only years before 2018 move. The default microsimulation years (2025 and 2026) do not change: misc_deduction is zero in both.

axiom: (pending)

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