Give Arkansas 2015-2020 income tax parameters their own values

[MaxGhenis]

Summary

policyengine_us/system.py runs backdate_parameters(first_instant="2015-01-01"), which copies each parameter's earliest value back to 2015. Most Arkansas income-tax parameters start in 2021 or 2022, so 2015-2020 Arkansas returns silently used 2021-2022 law.

This PR gives those parameters their own 2015-2020 values from DFA's forms and instructions, and fixes the one formula that depended on them. It follows #9611, which fixes the 2014-2020 rate schedules. The two PRs are independent and touch different files.

What	Before (backdated)	After (DFA)
Low income tax tables, all 7 files	2021 tables: higher zero-tax ceilings and tops	Every row of each year's tables, 2015-2020
Personal tax credit	$29	$26 for 2014-2019, $29 from 2020
Tuition deduction caps (AR1075)	2021 caps	2015-2020 caps
Unemployment compensation	Taxed in 2015-2017	Exempt through 2017; taxable from 2018 (Act 141 of 2017)
Military retirement	Fully exempt in every year	Before 2018, shares the $6,000 Line 17 exemption with pensions; fully exempt from 2018 (Act 141)
65 Special credit	Ignores military retirement	Before 2018, not allowed to a filer claiming the Line 17 exemption for military retirement
Additional tax credit for qualified individuals, inflationary relief credit	2022 amounts backdated to 2015. ar_files_separately reads the first credit's maximum in every year when it compares the joint and separate paths	0 before 2022, when they were created

Also fixes credits/personal/age_threshold.yaml's unit (currency-USD → year).

Examples

ar_income_tax, with only the main rate schedules. The #9611 fixes are not included, so these isolate this PR.

Household	Before	After
2015 single, wages $13,050 (low income table)	$0.00	$112.00
2018 joint, 2 children, wages $26,000 (low income table)	$0.00	$144.00
2016 single, wages $25,000 + unemployment compensation $8,000	$1,379.04	$1,093.83
2017 single, age 55, military retirement $30,000	$0.00	$1,033.83

How the low income tables were built

• Each year's DFA table was parsed twice, with two independent parsers: one from text layout and one from word bounding boxes. Every table-year agrees with a third parse by an independent reviewer: 30 of 30.
• With calc(agi, right=True), the model now equals DFA's table at every whole-dollar AGI from $0 to $45,000, for all 7 files and 2015-2020. That is 1.89 million comparisons with 0 mismatches, and inf above each table's top row.
  • The one DFA quirk is the 2017 head-of-household/qualifying-widow(er) table with 0-1 dependents. Its last row is printed "21,900-22,000", overlapping the previous row's 21,900, and the model keeps the earlier row's $574 at exactly $21,900.
• 2021+ is unchanged.
  • Brackets whose first value was dated after 2021 get an explicit 2021-01-01 value equal to their old earliest value, which is what backdating produced before.
  • New trailing brackets end in 2021-01-01: .inf.
  • The tables evaluated at every AGI from $0 to $60,000 for 2021-2026 are identical to main.

Tests

• low_income/ar_low_income_tax_joint_2015_2020.yaml: for each year, all 7 tables at the zero-tax ceiling, the first taxed row and the top row. Also a check that the table applies at the ceiling and not $1 above the top. The eligibility cases use a $30,000 taxable income so they fail on main.
• Cases added to the existing files for the personal credit (2015, 2019, 2020), the 65 Special credit (2017, 2018), tuition caps (2015, 2016, 2020), unemployment compensation (2016, 2017, 2018), military retirement (2016, 2017, 2018), and both 2022 credits before 2022.
• All but the boundary cases fail on main. The boundary cases are 2020's $29 credit, 2018's taxable unemployment compensation, and 2018's full military exemption.
• policyengine-core test on gov/states/ar plus contrib/states/ar: 541 passed. ruff passes.
• Three independent reviewers covered DFA data, 2021+ invariance and CI, and legal semantics with vectorized simulations. Their only finding against this branch, the 65 Special credit, is fixed here.

Not changed here

Already tracked in #9614, or pre-existing:

• The table qualifications for total income from all sources, military/pension exemption users, and itemizers.
• The Arkansas misc and casualty deductions.
• The partial-year 2015-2016 capital gains exclusion.
• 401(k)-type distributions before 2022 (Arkansas 2018 to 2021 exempts 401(k) and other employer-plan distributions that gross income never included #9580).

Separately, 2015-2016 households with a member 65+ cannot compute federal tax at all (#9613). The tests here supply inputs directly.

axiom: TheAxiomFoundation/rulespec-us#1405 debt

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[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 100.00%. Comparing base (909176a) to head (c152c04).
⚠️ Report is 49 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff            @@
##              main     #9615   +/-   ##
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  Coverage   100.00%   100.00%           
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  Files            4         2    -2     
  Lines           76        46   -30     
  Branches         2         1    -1     
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- Hits            76        46   -30     

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[MaxGhenis]

Heads-up from #9636 (Arkansas gambling winnings and losses): #9636 adds gambling_winnings to every period of gov.states.ar.tax.income.gross_income.sources.{joint,individual}. This PR adds a new 2015-01-01 block to those lists. Whichever PR merges second needs gambling_winnings in the 2015 block too; otherwise 2015-2017 silently drop gambling winnings from Arkansas gross income. The 2015 cases in ar_gambling_winnings_and_losses.yaml (from #9636) fail if that happens, so CI on the later PR will catch it.

[MaxGhenis]

Admin-merged (squash, --match-head-commit c152c04) under Max's 2026-09-26 ruling (d399: obvious improvements need no permission; admin-merge when the only block is a required review only Max could give). Gates at the reviewed head: gh pr checks exit 0, MERGEABLE, no CHANGES_REQUESTED. Approvals: an independent adversarial Opus 5.5 review of the final head (workflow wf_bfc37ebf, no blocker or major findings) plus the session-management main session's own diff review. Closes decision d287.